Kolarkar, Bitzios, Harvey, Carter
Risk Based Speed Limit Review and
Safety Management
Dr. PRAKASH N. KOLARKAR
Principal Engineer
Department of Transport and Main Roads
prakash.n.kolarkar@tmr.qld.gov.au
ABSTRACT
RISK BASED SPEED LIMIT REVIEW AND SAFETY MANAGEMENT
The Department of Transport and Main Roads Queensland (DTMR) has taken a pro-active approach
to managing the safety of the road network. The purpose of this paper is to share the knowledge
gained from pushing the boundaries including developing processes and systems that logically
enhance the overall safety of the road network.
The first part of this paper proposes a modified Speed Limit Review process based on the Safe System
Approach philosophy. It presents a holistic approach to the evaluation of speed limits by integrating
the findings of the road Safety Audit in the speed limit review process. The crash severity and crash
types along with the frequency of crashes are given significant weightage in classifying the speed
zones into extreme, high, medium and low risk sections.
The second part of the paper highlights the significance of a Centralised Database, developed to assist
in the identification of high risk intersections and mid-blocks in the network. The database integrates
the road safety audit and speed limit review findings to help break down an individual road corridor or
the entire network into segments based on levels of risk, thus assisting in the development and
prioritisation of safety programs.
Key Words: Speed limit, Safe System, Road safety, Crash severity.
1.
1.1
Speed Limit Review
Introduction
As a measure to improve road safety, DTMR is trialling a risk based approach in its speed limit review
process to determine the appropriate speed limit. The posted speed limit is an integral road parameter
which impacts many facets of a road operational performance. An appropriate speed limit will
provide enhanced road safety, efficiency and motorist compliance.
The assessment of the speed limit includes the identification of the relative risk of each distinct road
section. Each road section is reviewed in detail to assist in the development of speed zones and carry
out prioritised upgrades of road infrastructure wherever necessary. The intent of the process is to
ensure the road environment is consistent with the proposed speed limit. This includes consideration
of the crash history (including future crash potential) and the expectation for infrastructure
improvements.
The proposed speed limit review process is based on a combination of the Queensland MUTCD Part 4
guidelines and the safe system philosophy. The review is undertaken after completion of a Road
Safety Audit and is cognisant of its recommendations. Speed limit review recommendations are based
on a weighted crash risk matrix developed by DTMR. Any resulting action items are fully detailed;
including inventory of material required and sketches depicting works to be undertaken. These works
often (and in most cases) reference existing recommendations made within the Road Safety Audit and
where necessary, the Road Safety Audit is amended to incorporate the findings of the Speed Limit
Review.
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Risk Based Speed Limit Review and
Safety Management
1.2 Background
The primary purpose of setting speed limits is to advise drivers of the maximum reasonable and safe
operating speed that can be adopted under favourable conditions. It must also provide an acceptable
level of environmental amenity for all road users, abutting land owners and a reasonable basis for
enforcement.
Drivers continually make decisions about appropriate driving speeds by considering the amount of risk
they are willing to take. The prevailing or operating speeds resulting thereby are based on the
perceived risk of the driver. The perceived risk is almost always much lower than the actual risk, as
the drivers often lack sufficient information with respect to the capabilities of their vehicles, roadway
geometry, or other conditions to determine an appropriate speed. Andersson (2007) shows that the
median perceived risk of being killed in a road accident is about 60% of the actual risk of being killed
(i.e. people underestimate the risk). This places an increased level of attention required by road
authorities to ensure the road environment (through functionality and road design) is encouraging
travel speeds that are conducive to the ‘safe operating speed’.
Often, the driver’s inability to judge the effect of speed and a tendency to underestimate the effect of
speed on crash severity creates unnecessary risks for themselves and other road users. These
considerations suggest that prevailing speed may not be the best indicator of a reasonable or safe
speed. The prevailing 85th percentile speed has predominantly been used as the basis of decision
making for setting speed limits for many years. A number of manual and automated systems and
processes exist across numerous jurisdictions, however with most relying heavily on the existing 85th
percentile speeds as the key determining factor.
Crash statistics conservatively identify speeding, either in excess of the posted speed or simply
travelling too fast for the road conditions, as a major causal factor in around 30 per cent of fatal
crashes (Source: United States National Highway Traffic Safety Administration, 2012). The same
administration also highlighted that speed related fatalities on lower speed limit roads were reducing,
whilst higher speed roads were returning an increasing rate of speed related fatalities. Research and
evaluation showed that even a small decrease in travel speeds will lead to significant reductions in
crashes and the resulting trauma.
Apart from limited access motorways, Australia has relatively high speed limits across much of its
road network compared with the speed limits on similar roads in many OECD countries. Speed limits
on many of Australia and New Zealand’s roads are higher than those on comparable roads within the
Organisation for Economic Co-operation and Development (OECD) countries. For example local
roads in Australia are generally 50km/h, whereas many OECD countries have 30km/h or 40km/h
limits. Urban arterials are usually 60km/h, 70km/h or 80km/h. Limits for similar roads in OECD
countries are rarely more than 60 km/h.
In Australia, 110km/h is common on rural, undivided roads, however in many OECD countries speeds
above 100km/h are reserved for a motorway with full access control and rarely used on undivided
roads.
The safety standard of roads varies considerably and some may not be of an appropriate standard for
their current speed limit. Identifying and applying suitable speed limits are necessary measures to
improve safety. Alternatively, for high speed environments, infrastructure safety investment will be
needed to maintain ‘safe operating’ speeds. It is clear that a cultural change is required to alter driver
behaviours in Australia to reduce the expectation of what is a ‘safe operating speed’.
To further re-iterate the impacts speed has on crashes in Australia, the Australian Transport Council
(2006) reported that:
§
§
Speeds 5 km/h above average in urban areas and 10 km/h above average in rural areas double
the risk of a casualty crash;
small reductions in average speeds result in substantially greater percentage reductions in
deaths and injuries;
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Risk Based Speed Limit Review and
Figure 1: Change in mean speed vs. change in
Safety Management
crashes (Jurwicz, C. & Turner, B., 2011 based on
Elvik, 2009)
§
the chance of surviving a crash decreases dramatically above certain impact speeds for the
following types of crashes:
-
Pedestrian struck by a vehicle
Motorcyclist struck by vehicle or falling off Side impact on vehicle striking a pole or tree Side impact during vehicle to vehicle crash Head on vehicle to vehicle (equal mass) crash -
20 to 30 km/h
20 to 30 km/h
0 to 40 km/h
50 km/h
70 km/h
These observations are supported by Elvik (2009) who presented an update and new analyses of the
Power Model of the relationship between speed and road safety, based on a considerable dataset. The
research found that changes in speed have a smaller impact on road safety at low speeds than at high
speeds. This led to two versions of the Power Model being proposed:
§
§
rural roads and freeways to cater for high speed environment (refer Figure 1); and
roads in urban and residential
areas to cater for low speed
environment.
The proposed models confirm that the
number of crashes increases, with an
increase in speed and conversely, the
number of crashes decreases with a
decrease in speed. The study clearly
indicates that speed remains a very
important risk factor both for crash
occurrence and injury severity.
The meta-regression analysis used in
Elvik's (2009) study clearly indicates that
the effects of speed on road safety are
likely to be universal and not much
influenced by conditions that are specific
to a certain country.
In Methods and Practices for Setting
Speed Limits the United States Federal Highway Administration (FHWA) describes four primary
practices and methodologies that are used in setting of speed limits.
§
§
§
Engineering approach. This is a two-step process where the speed limit is set based on the
85th percentile speed i.e. the speed at which 85 percent of free flowing traffic is travelling at
or below (Operating Speed method) or the function of road (Road Risk method) and then
adjusted based on road and traffic conditions (such as pedestrian activity, median presence,
land use etc.) and crash history. In this method speed limits often get set below the 85th
percentile speed and may need enforcement or traffic calming measures to be employed.
Engineering approach based on 'Operating Speed' is popular in United States, whereas, the
approach based on 'Road Risk' is popular in Canada and New Zealand.
Expert System. In this process the speed limits are set by a computer program that uses the
knowledge and inference procedures that simulate the judgement and behaviour of speed limit
experts. This process is typically a computerised version of the engineering approach. In this
system the risk involved is that, review officers may tend to rely on output from the analysis
software and take decisions without applying a critical review of the results. It is popular in
United States and Australia.
Optimisation approach. Here the aim is to select a speed limit that would minimise the total
societal costs of transport. Parameters considered are travel time, vehicle operating costs, road
crashes, traffic noise, air pollution etc. The speed limits proposed may not be congruent with
the design of the road and thus attract non-compliance. The method is complex due to the cost
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§
Risk Based Speed Limit Review and
Safety Management
models and extensive input data required for the development of optimal speed prediction
models and thus rarely used.
Injury minimization or safe system approach. Here, the speed limits are set according to the
crash types; the impact forces that result, and the human body's tolerance to withstand these
forces. The method establishes a sound scientific link between speed limits and serious crash
prevention. This approach takes the stand that it is unethical to create a situation where
fatalities are a likely outcome of a crash in order to reduce delay, fuel consumption, or other
societal objectives. Thus vehicles cannot legally travel at speeds where, in the event of a crash,
the impact can result in a serious or fatal injury. This method is popular in Sweden and
Netherlands.
1.3 Setting Speed Limits in Queensland
In Queensland, the legal provision for speed limits is outlined in Part 3 of the Transport Operations
Road Use Management Regulation 2009 (TORUM). The methods, standards and procedural
requirements are detailed in Part 4 of the Manual of Uniform Traffic Control Devices (MUTCD).
In Queensland, speed limits are set in accordance with Part 4 of the Manual of Uniform Traffic
Control Devices (MUTCD). Use of the MUTCD is required to ensure that speed limits are consistent
and credible throughout the state.
There are three key assessment criteria to assess existing speed sections which are:
§
§
§
Assessment of Road Function, which is the road’s role within a road network and usually
planned before a road’s construction however this may change over time due to land
development;
Assessment of Prevailing Traffic Speeds. Prevailing traffic speeds are usually representative
of the general perception of a reasonable travel speed on a particular section of road; and
Assessment of the Speed Environment, determined by the QLIMITS Software using
parameters such as roadside development, road characteristics and traffic characteristics.
The QLIMITS software package considers the recommended speed limits in accordance with MUTCD
guidelines. As detailed earlier, this is a three stage assessment incorporating road function, prevailing
traffic speed and speed environment. The analysis software will select a speed limit that represents
correlation of recommended speed limits from a minimum of two of the three stages.
Both the Queensland MUTCD and QLIMITS list several risk factors including road geometry,
roadside development, parking, pedestrian activity, crash rate, etc., however, they do not provide any
specific guidance on how to cater for these variables in the setting of speed limits.
The current practice of conducting speed limit reviews relies heavily on road function, prevailing
speeds and the evaluation of road and roadside environment using an analysis based software. Most
reviews are desktop reviews and little weightage is given to intersection crashes, crash trends and
other significant safety issues which have a potential to result in high severity crashes (such as design
standard of the road, specific location of hazards within the clear zone, specific location of unique
adjacent land uses, local driver behaviour or driver demographic related issues).
The Queensland MUTCD however does state that if a high crash rate exists and/or a road safety audit
has been completed then it may warrant a review of the QLIMITS findings. DTMR has taken this
process further by developing processes to comprehensively detail and quantify the Road Safety Audit
and Crash Risk findings. This is a key component of the proposed improvement of existing processes.
Crash severity and crash potential, by a common crash type, are generally not well catered for in the
Speed Limit assessment process. Also, QLIMITS is not an adaptive or ‘smart’ system, as the logic is
hard coded, it does not learn from previous experience thus making it imperative for the analyst to use
engineering judgement to make proper decisions. For instance, post-implementation data of crash
rates or prevailing travel speeds are not recorded to refine recommended speed settings. The system
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Risk Based Speed Limit Review and
Safety Management
also does not evaluate / consider the design standard of the road (i.e. length of turn pockets,
merge/diverge area, number and proximity of roadside objects, etc.).
The subjectivity involved in the process of setting speed limits, can lead to inconsistencies in posted
speeds within and between jurisdictions making it hard to justify and defend when challenged. In
addition, the reviewing officer has the ability to re-assess the speed limit if the initial output was not
considered appropriate. With a number of the measures being qualitative, the ability for an assessor to
choose a desired speed limit is still possible. Under the current speed limit review process substandard
sections are generally ignored if short in length, as multiple speed zones are undesirable.
In essence, the setting of speed limits is a critical component to the assessment of the road safety
aspects of a road section. Road agencies have the challenging decision to reduce the speed limit and
alter the road environment to suit or to maintain the existing speed limit and address all safety
shortfalls adopting the Safe System Approach to bring the road environment up to the operating speed
standard.
The Safe System Approach involves a methodology based on best international practice, and consists
of four key aspects as outlined below.
1) Safe Roads and Roadsides
Roads and roadsides should be designed and maintained to reduce the risk of crashes occurring and
to lessen the severity of injury if a crash does occur. Safe roads prevent unintended use through design
and encourage safe behaviour by users.
2) Safe Speeds
Speed not only determines the likely risk of a crash but also the outcome of the crash or severity.
Lower speeds result in fewer crashes as road users have more time for decision making, are less likely
to lose control and can stop within a shorter distance. Speed limits complementing the road
environment should be implemented to manage crash impact forces to within human tolerance; and all
road users complying with the speed limits.
3) Safe Vehicles
The introduction of vehicles which not only lessen the likelihood of a crash and protect occupants, but
also simplify the driving task and protect vulnerable users. Increasingly this will involve vehicles that
communicate with roads and other vehicles, while automating protective systems when crash risk is
elevated.
4) Safe Behaviours
Encouragement should be given to safe, consistent and compliant behaviour through well-informed
and educated road users. Licensing, education, road rules, enforcement and sanctions are all part of
the Safe System.
DTMR recognises the objective of the National Road Safety Strategy 2011-2020 to reduce road
trauma by 30 per cent. The safe system approach as conceptually referred to in Austroads is shown in
Figure 2.
Figure 2: Safe System Approach (Austroads, Guide to Road Safety – Part 1)
The Safe System Approach recognises safe speed, which is more forgiving of human error, as one of
the key parameters in reducing road trauma. However, the conventional speed limit review process
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Risk Based Speed Limit Review and
Safety Management
does not guide assessors to the concept of safe speed to influence the setting of speed limits on
existing roads.
The DTMR trial proposes a ‘modern approach’ to setting speed limits. It has considered two of the
key aspects: safe roads (and roadsides) and safe speeds. To take into consideration the ‘safe system’
approach DTMR developed a qualitative risk-based system to determine the appropriate speed limit.
The assessment of speed limit included the identification of the relative risk of each distinct road
section reviewed.
The principle of harm minimisation in the context of a ‘Safe System Approach’ limits the vehicle
speeds to suit the available road infrastructure such that severe or fatal injuries are eliminated. To
achieve this objective, simply lowering speed limits to the level of safe speeds as recognised by the
harm minimization principle is likely to receive low compliance, whilst the provision of infrastructure
alone to close the gap between the calculated safe speed and the existing posted speed on the road, is
unacceptable in the current funding scenario and would only be possible as a long term goal.
An Austroads paper prepared in 2005 titled “Balance between Harm Reduction and Mobility in
Setting Speed Limits: A Feasibility Study” confirmed that most road authorities in Australia felt their
speed limits were set too high and that reduced speeds would have substantial safety benefits. It also
highlighted that the travel time dis-benefits from a reduced speed limit are likely to be outweighed by
the safety benefit. A suggestion made in the paper was that “A safety profile, based on crash history
and/or road-related risk factors could be developed for each road or road segment, whereby the
appropriate default speed would be implemented if (and only if) the profile achieved at least the
minimum specified safety measures. Failure to achieve a minimum safety profile would result in a
lowered speed, either for the whole road or for specified segments. Conversely, a road or road
segment with a good safety profile could be awarded a posted speed higher than the default value.”
DTMR trial proposes a harm reduction approach whereby the recommendation in high risk road
sections of the network would be to set speed limits to values slightly lower than those suggested by
the road class and function of the road but higher than the safe system speed, along with consideration
of the works required to improve road infrastructure in the short term to medium term. This would
ensure a smaller, potentially consistent and practical drop in recommended speed limits that would
have better acceptance by the public when supported by advisory/warning signage along with a
prioritised program of works to improve the road infrastructure in the short term based on the
availability of funds.
DTMR trial has extended the Safe System Approach framework to provide more detailed processes in
the sub-area of “Understanding Crashes and Risks”.
The two components of risk used in the assessment were frequency and severity. The frequency of the
crashes relates to traffic volumes, speed, road width and cross section which included clear zone
hazards, road curvature, intersection frequency, and roadside activity. Severity is related to the type of
crash that is likely to occur and include factors such as the angle and speed of collision, and type of
hazard struck.
The scope of work typically included in a Speed Limit Review for the DTMR’s trial is as follows:
§
§
§
§
§
§
§
Review based on the road safety audit findings, MUTCD Part 4 guidelines and safe system
approach;
Community concerns, feedback, complaints
Identification of the road segments that have high crash rates;
Categorise risks for road segments as low, medium, high or extreme priority based on a risk
matrix, incorporating crash and traffic volume data;
Brief impact study on crash rates of any road improvement works previously undertaken,
including signalisation or addition of turning lanes to intersections, where available
Prioritised and detailed recommendations for road segments needing upgrading.
Recommendations based on a risk matrix and improvement of the safe speed threshold (based
on the Safe System approach). Any resulting action items are to be fully detailed; including
inventory of material required and sketches depicting works to be undertaken.
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Risk Based Speed Limit Review and
Safety Management
The current DTMR process combines the Road Safety Audit and Speed Limit Review process which
provides the audit team with an in depth understanding of the study section, including the specific
details of each crash cluster, road safety and traffic operations issues. This enables road safety
advisors to be empowered with the required detailed local data to make informed decisions as to the
appropriateness of a recommended speed zone.
1.3.1 Overview of Quantitative Assessment
In order to pro-actively rank the crash risk associated with each road section a quantitative assessment
methodology has been developed. The process quantifies the crash frequency and crash risk to
develop a “Crash Risk Score (CRS)”. As it is difficult to identify the role of speed in crashes, speed
related crashes are often grossly underreported, and hence all crashes are considered in the setting of
the speed limits.
Crash Frequency: The quantitative measures adopted for the crash frequency is the ‘crash rate’ (i.e.
crashes per VKT x 108). A minimum 1km road section length is desirable for this calculation to
reduce distance effects on the crash rate.
Crash Severity: The crash severity is somewhat catered for by the crash types quantified in developing
the crash rates. However, these rates have been developed over a much broader data set and do not
represent the local crash history associated with the study section. The severities of the crashes along
the study section in each DCA group were quantitatively assessed in order to assign a ‘severity’ rating.
Adopting the ‘crash cost’ to determine a quantitative measure for ‘crash severity’ was given
consideration, however, the relative difference between a ‘fatality’ and all other crashes, presented an
unrealistic relationship between the comparative value of ‘severity’ placed between these crash types.
In particular, the difference between a ‘fatality’ and a ‘hospitalisation’ occurring can be often marginal
and a monetary based weighting is unlikely to provide a true indication or representation of the ‘crash
potential’.
A “squared” growth function was considered to present a more realistic relationship to quantify
‘severity’, particularly from an agency ‘need to respond’ perspective. Following crash scores have
been adopted for each level of rash severity:
§
§
§
§
Property Damage Only
Minor Injury / Medical Treatment
Hospitalisation
Fatality
–
–
–
–
Score = 1 (Limited Severity)
Score = 4 (Minor Severity = 22)
Score = 9 (Serious Severity = 32)
Score = 16 (Catastrophic Severity = 42)
The above weightings will be monitored over time to review their effectiveness. Early indications
suggest that the above factors are providing realistic and representative results in determining the level
of crash risk associated with a given road section.
1.3.2 Total Crash Risk Score (CRS)
The total crash risk score (CRS) for each DCA group was attained from multiplying the Crash Rate
per VKT^8 for each crash severity by the relevant crash severity rating for any speed limit assessment
road section. The multiplication process is done for each severity separately, each with its own ‘Crash
Rate per VKT^8’ and ‘Severity Score’, then added together to obtain a total CRS.
For example:
Crash Risk Score = CR1 (1) + CR2 (4) + CR3 (9) + CR4 (16)
Where CR1, CR2, CR3 and CR4 are crash rates for PDO, MI/MT, Hospital and Fatal crashes respectively
The final crash scores obtained were categorised into Low/Medium/High/Extreme crash risks
following the risk matrix described in Table 1.
Table 1: Crash Risk Score Categories (Mt Cotton Rd Speed Limit Review, DTMR and Bitzios
Consulting, 2014)
CRASH FREQUENCY (Crash Rate per VKT^8)
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S
E
V
E
R
I
T
Y
Improbable (I)
[1]
Low
[1]
Low
[4]
Medium
[9]
High
[16]
Limited (PD)
[1]
Minor (MI / MT)
[4]
Serious (H)
[9]
Catastrophic (F)
[16]
Risk Based Speed Limit Review and
Safety Management
Occasional (O)
[4]
Low
[4]
Medium
[16]
High
[36]
Extreme
[64]
Probable (P)
[9]
Medium
[9]
High
[36]
Extreme
[81]
Extreme
[144]
Frequent (F)
[16]
High
[16]
Extreme
[64]
Extreme
[144]
Extreme
[256]
The final CRS thresholds adopted to arrive at a final crash risk rating are as follows:
§
§
§
§
Low Risk
Medium Risk
High Risk
Extreme Risk
–
–
–
–
0 ≤ CRS < 7
7 ≤ CRS < 16
16 ≤ CRS < 50
CRS ≥ 50
1.3.3 Capturing the existing conditions, risk level and treatment options
The application of the weightings for severity and frequency outlined in the matrix to the risk
assessment process for each road section and crash type allows identification of treatments with a
direct link to increased safety. The resultant suggestion can be broken down into the following
categories:
§
§
§
§
Change speed limit;
Upgrade infrastructure;
Interim low cost treatment; and
A blended response (combination of the above).
The aim of the blended responses is to reduce harm through practical short term solutions. This is
most advantageous at locations where infrastructure upgrades are not feasible. Table 2 below is an
example of the risk assessment table. The table determines the existing condition risk and indicates
how proposed actions (works, speed reduction, etc.) will alter the risk levels at this location for a
particular crash type. Left side of the table shows the road section, crash type and crash rate. This is
followed by the existing condition risk assessment and QLIMITS recommendation. On the right is the
action and corresponding risk assessment.
1.3.3.1
Example: Moggill Rd Speed Limit Review Risk Assessment (refer table 2)
In this example it can be seen that for crashes of DCA Code 200-209 the risk has been reduced, based
on having an improved understanding of the specific nature of the crash (i.e. driver was on medication
and the road did not contribute to the crash). For the 300-309 (rear end) type crashes, the speed limit
over the section was recommended to be retained at 60km/h through the MUTCD/QLIMITS process.
However, the possibility exists to take action that would reduce the risk by re-configuring the
approach lane arrangements to support the local operating conditions.
This improvement would lower the likelihood of this crash type occurring, reducing frequency from
‘probable’ (P) to ‘occasional’ (O) and the subsequent overall rating to ‘medium’.
Where the overall crash risk for a crash type occurring over an assessment section is determined to be
‘extreme’ it is often warranted that the speed limit can be reduced on safety grounds. One example of
this is currently in use in the USA. USLIMITS2, a web based expert approach used for setting of
speed limits in the United States, recommends the 50th percentile or median speed when the crash risk
is significantly higher than the average for similar roads, or when other risk factors such as significant
pedestrian activity is present. Whilst this process hasn’t been formally embedded within the DTMR
process it is given strong consideration when assessing the merits of reducing speed limits.
Table 2: Crash Risk Table (Moggill Rd Speed Limit Review, DTMR & Bitzios Consulting 2014)
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Risk Based Speed Limit Review and
Safety Management
Figure 3: Speed zones and risk for road section by
colour (Mt Cotton SLR, DTMR & Bitzios
Consulting 2014)
It is worth noting that lower speeds alone do
not guarantee a safe operational road
environment. Speed limits must be perceived
as rational and appropriate by drivers to
ensure satisfactory compliance. If this is not
the case, an increase in crash rate may occur
due to increased variance in travel speeds.
As a final output of the speed limit review
process, the existing crash risk levels
according to the developed risk matrix is
overlayed on a map to clearly depict the high
risk zones along a route (refer Figure 3).
These overlay maps are very useful to
identifying trouble areas along road sections
for further interrogation or when advice is
requested for potential future projects.
2.
Safety Investigation and
Centralised Database
2.1 Introduction
This section of the paper gives the approach adopted by the DTMR trail for carrying out safety
investigations and the use of an in-house developed database to assist in data analysis, identification of
high risk sections in the network and in the prioritisation of works for delivery.
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Risk Based Speed Limit Review and
Safety Management
2.2 Background
Australia’s first National Road Safety Strategy was established by federal, state and territory Ministers
in 1992. Australia was one of the first countries to formally adopt the Safe System approach to road
safety improvement under the 2001 - 2010 strategy, which aimed to achieve a 40% reduction in the
per capita rate of road deaths. An actual reduction of 34% was achieved by 2010. This reduction,
although less than the set target, was still commendable. The National Road Safety Strategy (NRSS)
2011-2020 has gone a step further and has presented a 10 year plan to reduce the annual number of
deaths and serious injuries by at least 30% as a step towards the long term vision based on safe system
approach that no person shall be killed or seriously injured on Australia's roads.
Funding availability is limited, funds are only available for a select number of major infrastructure
projects, and funds are limited for routine maintenance and very minor works (low cost works). It is
critical that if any funding becomes available suitable projects can be quickly and appropriately
identified. With this being the case, DTMR have focussed on developing a Centralised Database
System that enables the authority to be responsive in providing candidate projects of varying size, with
appropriate justification based on a Speed Limit Review and Road Safety Audit recommendation. The
system has also been very useful for Black Spot and Safer Roads Sooner funding submissions,
providing overall human resource cost efficiency to the State.
2.3 The DTMR Proactive Approach
DMTR have had to ‘think outside the square’ in developing a Centralised Database which ticks many
boxes in working towards achieving safety targets identified in the National Road Safety Strategy
(NRSS). It is a proactive response to maintenance, audits and crash investigations. Over the past 10
years maintenance practices have become more targeted. The DTMR Road Safety Audit processes
capture the conditions of the road network through inventory checklists. These inventories are also
used to assist with implementing Mass Action Programs to enhance safety of a road section. The
Centralised Database has created a philosophical change within DTMR, whereby road safety no longer
resides on its own. The Centralised Database is used to store Road Safety Audit findings as well as
network deficiencies captured in the inventories to proactively manage the condition of safety critical
asset items such as signs, lines, delineation, guardrail, vegetation and roadside objects.
DTMR also has in place a serious/fatal crash investigation program, which addresses the risks
identified at crash locations and follows up aggressively with the investigation of suitable
implementation of recommendations to improve safety in the region. The Road Safety Audit process
developed makes it possible to identify and produce a list of low cost-high benefit treatments to
address deficiencies across the transport network which results in targeted, cost-effective treatments.
Black Spot Management (BSM) has been found to be very effective in DTMR for the treatment of
specific locations having a high severity crash rate. To supplement BSM, DTMR has adopted a
broader and more strategic approach to identify and treat other high risk locations and crash cluster
sites under its Safer Roads Sooner (SRS) program (popularly known as Network Safety management
(NSM) in most countries) and include lengths of roads that have a history of serious casualty crashes
(Black Links) or possess a high potential for serious injury type crashes based on local road conditions
as identified by road safety audits. The SRS program addresses longer road sections up to 10 km,
whereas the Black Spots are generally up to 0.5 km in length.
To help achieve the 30% reduction in fatal and serious injury crashes as set out in NRSS 2011-2020,
DTMR has aggressively pursued its Road Safety Audit and Speed Limit Review programs, to identify
safety issues in the network. More than 70% of the State road network within Queensland, has been
reviewed under this effort by DTMR in the last 4 years. In DTMR the successful roll out of the
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Risk Based Speed Limit Review and
Safety Management
prioritised program of works following the implementation of the safety programs has been effective
in greatly reducing the number of new Black Spots emerging in the network.
To support the above data collection, analysis and storage a Centralised Database has been set up in
DTMR to carry out analysis and reporting of crash clusters and identification of high risk segments in
the Network to help in the development of Black Spot and Safer Roads Sooner business cases and
target infrastructure upgrades at high risk locations in the Network. The database is capable of
screening the network and sorting crash data over a specified period of time by severity, DCA group
and location to identify crash trends in fixed sections of 100m or 2km as desired. This is a different
approach to the identification of black spots in for example Austria which uses the sliding window
approach, where a window of specific length is moved along a road to flag locations meeting the black
spot criteria (Austrian Guideline Code for Planning, Construction and Maintenance of Roads 2002).
The use of the sliding window approach in the identification of black spots has been discouraged as it
is noted to artificially inflate the number of black sections, and make each section look blacker than it
really is (Sorensen and Elvik, 2007). This has been shown empirically by Elvik (1988a, 2007).
Theoretical work carried out by Hauer and Quaye (1990, 1993) confirm that using a sliding window
greatly inflates the number of false positives as black spots.
Alternatively, if fixed sections are used, there is a possibility that local accident peaks might get
divided between two segments and may not get picked up as a black spot, giving a false negative. The
problem can be avoided by reducing the segment length, but this increases the risk of random accident
peaks being identified as black spots (Hauer et al. 2002).
DTMR has adopted a dual fixed sections approach, with the Centralised Database giving the ability to
select segment sizes of 100m or 2km as desired. The sites identified can be further filtered to weed out
the false positives associated with random peaks, as the database can generate graphs to analyse the
crash clusters and assist in the identification of possible trends.
The Centralised Database also helps, in the risk ranking of the intersections and mid-blocks in the
network. The ranking can be based either on total number of crashes, total DCA cost, crash severity or
DCA group to target specific mass action programs, like the wide centre line treatment adopted to
reduce head on collisions in mid-blocks, guard rail installation/upgrade in locations high in off-road
crashes, skid resistance improvement in sections high in wet weather crashes etc.
The BSM works identified by the Centralised Database are typically remedial and retrospective in
nature as they are based on crash history. However, the SRS or NSM works identified by the database
after screening the Road Safety Audits and Speed Limit Reviews for each road in the Network has
potentially both a remedial and retrospective nature and a preventive and prospective nature as it is
based on both crashes and identified network deficiencies. It is this pro-active component that DTMR
is actively focusing on to help close the gap between the safe and posted speeds along the hazardous
sections of the road network. This is to achieve better alignment with the Safe System approach
philosophy.
A critical part of the Road Safety Audit process is to include the sites identified by the Centralised
Database as black spots or hazardous links for assessment during the review process. The detailed
inspection during the auditing process verifies the validity of the site as a true black spot or high risk
segment. It also helps to identify other safety issues that escape the crash cluster analysis, thus adding
to the process a preventive and prospective nature. As the road safety audit is a formal process, it adds
objectivity and completeness along with a clearly defined reporting format to ensure proper
documentation of all the safety issues and deficiencies identified. The road safety audit attempts to
determine the crash factors explaining why the crashes occurred as well as the injury factors detailing
why the crashes became serious, which assumes significance in the Safe System approach
environment. The recommended treatment will then attempt to either eliminate or mitigate the crash
and injury factors. The recommendations will receive a higher priority if the local risk factors are
leading to serious injury crashes (i.e. not just crash history based and considers the crash potential).
All the recommendations for individual roads and for the network can be filtered by priority and
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Risk Based Speed Limit Review and
Safety Management
category using the Centralised Database for targeting infrastructure upgrades and development of
Black Spot, Safer Road Sooner or category specific mass action programs like guard rail upgrades,
drainage improvement, cycle safety, pedestrian safety etc.
2.4 System Integration
The overall broader integrated process of the Road Safety Audit and Speed Limit Review is
generically shown in Figure 5. The intent of the process is to enable a pro-active approach to
responding to a network of Road Safety Audit/s, Speed Limit Review/s and Crash Investigation/s
findings. Figure 4 is presented to demonstrate that a series of Road Safety audits occur along a
number of road corridors and road sections.
As the road authority responsible for the management of the main road network, the outputs from each
individual assessment are combined into a singular database where the activities required from a
program manager can occur in an efficient and accurate manner.
Consistent findings across the transport network can also lead to policy and non-infrastructure based
decisions. Essentially, the process is providing a level of detail that has never previously existed in
relation to road safety. Issues, that anecdotally were considered crash trends across the DTMR
transport network, can now be justifiably quantified.
Figure 4: Integrated Road Safety Audit and Speed Limit Review Framework (Mt Cotton Rd
Speed Limit Review DTMR and Bitzios Consulting, 2014)
2.5 DTMR Centralised Database – Structure and Processes
DTMR undertakes a rolling program of road safety audits. The recommendations from each audit can
number into hundreds. All recommendation are recorded, prioritised and actioned using the
Centralised Database. Once audit recommendations are actioned the implementation outcomes are
also included in the database. The Centralised Database provides the following abilities:
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Organise and sort audit recommendations by location, work type and element;
Assists with the development of works orders;
Tracking of the status of reviews and recommendations;
Easily accessible information regarding reviews and works undertaken;
Recording and reviewing costs of safety works;
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Risk Based Speed Limit Review and
Safety Management
Enables recording and review of quality and appropriateness of audit recommendations (i.e.
issues unforeseen by auditors or recommendations that were ultimately unsuitable);
Assist in development of business cases & mass action programs;
Assist in crash cluster detection, analysis & reporting; and
Assist in risk ranking of crashes at midblock & intersection locations.
The databases tabular and graphical interfaces enable simple interrogation of the data. In an
environment of limited resources, this enables officers to prioritise safety works from across the
DTMR network and ensures the prioritisation is based on sound, consistent and measured application
of road safety principles.
2.5.1 Development of element works programs
Targeted works programs can be developed using the sorting functionality of the database. Works
items can be sorted by detail such as road, chainage, work type (elements), priority and cost.
The databases ability to filter issues are shown in Figure 5. Items can be filtered by road, chainage
and work type. Results can also be easily exported into an excel spreadsheet for forwarding on to
delivery units of DTMR.
The issues database contains all road safety audit recommendations, sortable by element classification.
The database also allows input of feedback from element leaders and delivery units regarding the
implementation of recommendations, for example implementation dates and construction costs. This
allows monitoring of delivery performance and costs (estimates versus construction).
2.5.2 Cluster Analysis
The Centralised Database can provide crash data analysis to identify crash trends (clusters), sortable
by multiple parameters including DCA Group, location, weather, time of day, etc. This enables rapid
identification of trends and patterns at any location of the network.
Crashes can be identified across 100m or 2km sections. The example shows an analysis of Head-on
type crashes (DCA Code 2). Figure 6 shows an example output from the Centralised Database.
Locations identified can then be investigated for suitability for remedial works, such as Wide Centre
Line Treatment, Guardrail Treatment, Minor Signs, RRPM/Guideposts, etc.).
2.5.3 Speed Limit Zoning
DTMR is involved in all aspects of setting and maintaining the speed limits on State Controlled Road
within their jurisdiction. The region also undertakes a proactive rolling program of speed limit
reviews to ensure ongoing appropriateness of speed limits. The Centralised Database centrally stores
all information pertaining to these efforts, including review reports, modifications to speed zones,
correspondence with the speed management committee and work orders. Mapping of the network is
available by speed zone through the integration of Google maps.
2.5.4 Highlights regional priorities
The region endeavours to fast track engineering treatments at high risk sites on the state-controlled
network. Treatments will comprise high benefit, low cost safety improvement projects targeting
serious crash types, including head on, run off road and crashes on high speed roads (Queensland
Road Safety Action Plan, 2013-2015 – DTMR). The Centralised Database aides this function by
identifying sites, prioritising recommendations and identifying clusters, and tracking road safety
funding submissions. This is of assistance with the following projects: Safer Roads Sooner; Black
Spot; mass action program; cycle safety; killed or serious Injury (KSI); unfunded and future
submission projects.
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Safety Management
2.5.5 Impact Study
The Centralised Database enables users to identify and measure the effectiveness of road safety related
activities through the reduction of the occurrence and/or severity of crashes at a particular location.
This is achieved through comparison the crash history in periods before and after the works was
implemented. Over-time, more accurate benefit-cost ratios (BCR) will be able to be established for
road safety related activities. This will provide an effective tool to better understand the difference
between effective and ineffective treatments and whether process solutions in fact addressed the crash
risk, or potentially created a new risk.
2.5.6 Business Case Development
The Centralised Database assists in the development of business cases and mass action programs
through sorting crash data by DCA code, crash severity and crash type. In Figure 7 a cluster analysis
can be seen for DTMR Road U14. This yields crash numbers by DCA group, severity and total crash
costs. These columns can be sorted according to the DCA codes of concern, resulting in the
identification of clusters and also indicating their relative ranking on the wider network.
Figure 5: Example Filtering of Options (DTMR, 2014)
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Risk Based Speed Limit Review and
Safety Management
Figure 6: Example of Crash Data Filtering (DTMR, 2014)
Figure 8: Output of Fatal Crash Sites within
Metropolitan Region, QLD (DTMR, 2014)
Figure 9: Intersection crash cluster graphs
(DTMR, 2014)
Figure 7: Example Output of Mass Action Program development (DTMR, 2014)
2.5.7 Interactive Mapping
The Centralised Database interfaces and overlays a variety of data on Google maps to graphically
display outputs. This includes fatal crashes, segment sites, speed zones and high crash zones. New
maps can be produced as the database is updated. Figure 8 provides an example of a typical output
provided through the Centralised Database System.
Reliance on the road safety audit process to rank issues / treatments by priority is considered to
contain sufficient detail to rank deficiencies across the corridor. The road section risk quantification
process based on the crash history as well as consideration of road safety audit findings provides a
sound basis for prioritisation - given the financial realisation that projects are justified in most cases
based around ‘return on investment’ calculations. Opportunities exist within various funding streams
to justify works based on a proactive crash prevention strategy; however these funding opportunities
are few and far between and cannot practically be used as the sole basis for justifying projects for
implementation.
Network maps showing crash clusters or crash trends can be produced using data extracted from the
database using the search functionality. The database can also produce crash cluster analysis by
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Risk Based Speed Limit Review and
Safety Management
location, such as at intersections and display the data graphically using graphs and maps, as shown in
Figure 9 above.
2.6 Concluding Remarks
DTMR has undertaken a strong commitment to reducing the road toll on the State Controlled
Network. Whilst the integrated proactive approach to road safety has been established as a trial, the
region as a whole has provided total support to the program by way of both resource commitment and
follow-up infrastructure funding commitment.
The integrated proactive approach does not modify the current approved guidelines in setting Speed
Limits in Queensland or for conducting Road Safety Audits as outlined by Austroads, but rather aims
to further extend the learnings gained from a variety of sources to increase the success rate for the
implementation of road safety based programs.
DTMR has already started noticing the network management benefits and the ability to be more
responsive in developing project schemes as funds become available. It also enables the delivery of a
reliable method for accurately producing project schemes that are addressing the issues at hand. More
importantly, the systems and processes are being developed to enable ongoing performance
monitoring of both the Centralised Database system and the transport network.
The tools and process’ developed in the trial will be considered for further use within DTMR where
resources and data are available.
3.
References
1) Andersson, H. (2007). Willingness to pay for road safety and estimates of the risk of death:
Evidence from a Swedish contingent valuation study. Accident Analysis and Prevention, 39, 853865.
2) Austrian Guideline Code for Planning, Construction and Maintenance of roads – RVS 1.21
(2002).
3) Austroads Guide to Road Safety, Part 1: Road Safety Overview, 2006.
4) Austroads Guide to Road Safety, Part 3: Speed Limit and Speed Management. 2006.
5) Austroads Guide to Road Safety, Part 7: Road Network Crash Risk Assessment and Management,
2006.
6) Balance between Harm Reduction and Mobility in Setting Speed Limits: A Feasibility Study,
Austroads – 2005.
7) Elvik, R. (2009). The Power Model of the relationship between speed and road safety: update and
new analyses, TOI Report 1034:2009. Institute of Transport Economics, Oslo, Norway.
8) Elvik, R. (2007). State of the art approaches to road accident black spot management and safety
analysis of road networks. Report 1 of works package 6 of RIPCORD-ISEREST.
9) Elvik, R. (1988a). Some difficulties in defining populations of “entities” for estimating the
expected number of accidents. Accident Analysis and Prevention, 20, 261-275.
10) Federal Highway Administration. Methods and Practices for Setting Speed Limits (FHWA-SA12-004). Washington, DC 2012.
11) Hauer, E.,J.Kononov, B. Allery and M. Griffith (2002). Screening the Road Network for Sites with
Promise, Transport Research Record, 1784, 27-32.
12) Hauer, E., K. Quaye and Z. Liu (1993). On the use of accident or conviction counts to trigger
action. Transport Research Record, 1401, 17-25.
13) Illinois Department of Transportation, Policy on Establishing and Posted Speed Limits on the
State Highway System (Mar 2011).
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Safety Management
14) Jurewicz, C. & Turner, B. (2011). Risk-based approach to speed limits - a step towards safe
system, Australasian College of Road Safety Conference "A Safe System: Making it Happen!"
Melbourne 1-2 September 2011.
15) Manual of Uniform Traffic Control Devices Part 4 Speed Controls (2003). DTMR.
16) Moggill Road, Road Safety Audit and Speed Limit Review, DTMR and Bitzios Consulting, 2013.
17) Mount Cotton Road Speed Limit Review, DTMR and Bitzios Consulting, 2014.
18) National Road Safety Strategy (2011)
www.infrastructure.gov.au/roads/safety/national_road_safety_strategy/files/NRSS_2011_2020_1
5Aug11.pdf
19) Transport Operations (Road Use Management – Road Rules) Regulation 2009 – Part 3, DTMR
Queensland.
20) Queensland Road Safety Action Plan 2013-2015. DTMR
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