Lec 33, Ch.5, pp.147-164: Accident
reduction capabilities and effectiveness of
safety design features (Objectives)
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Learn what’s involved in safety engineering
studies
Learn how to compute accident reduction
capabilities of countermeasures
Learn how to estimate the effectiveness of
safety design features (Reduction of the
number of accidents)
What we discuss in class today…
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Components of engineering studies
Condition diagram and collision
diagrams
Accident reduction capabilities of
countermeasures
Accident reduction factors – definitions
Accident reduction factors relating to
improvements to roadway cross section
Component
of the
Highway
Safety
Improvemen
t Program
(HSIP) by
FHWA
Need estimates of
the effectiveness of
safety design
features
Conducting engineering studies (after
hazardous locations have been identified)
Steps:
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In-depth study of the accident data obtained
for the study site
Conduct a field review of the study site
List possible accident (contributory) causes
Determine specific safety deficiencies at the
site
Develop general countermeasures
Conduct an economic analysis (costeffectiveness, rather than cost-benefit)
Recommend a list of countermeasure actions
Site analysis – Draw a condition diagram
Purposes:
 To identify contributing causes
 To develop site specific improvements
Two types of info:
Accident data
Environment & physical
condition data
The first thing you do
is visit the site and
prepare a condition
diagram of the site.
Site analysis
(cont) –
Prepare a
collision
diagrams
Site analysis (cont) – Questions to ask
Group accidents by type and answer the following 3 questions,
which will lead you to possible countermeasures. See Table 5.3.
 What driver actions led to the occurrence of such an accident?
 What conditions existing at the location could contribute to
drivers’ taking such actions
 What changes can be made to reduce the chance of such
actions occurring in the future?
Rear-end collisions:
Driver: Sudden stop & Tailgating
Environment: Too many accesses and interactions with vehicles in/out of the
accesses (drive ways), bad sight distance, short/long yellow interval, inappropriate
location of stop lines (against driver expectancy), etc.
Crash reduction capabilities
 Used to estimate the expected reduction in crashes that will
occur during a given period as a result of implementing a
proposed countermeasure.
 CR = crash reduction (CR) factors are used to indicate
potential crash reduction capabilities.

ADT _ after _ period 
Crashes _ prevented  N  CR
 ADT _ before _ period 
N = expected number of crashes if countermeasure is not
implemented and if the traffic volume remains the same.
Example 5-5:
CR = 0.3, ADT before = 7850, ADT after = 9000, No. of specific types
of crash occurring per year = 12, 14, 13 for the same 3 years where
ADT average values were computed.
Avg no. of crashes/year = (12+14+13)/3=13
Crashes prevented = 13 x 0.3 x (9000/7850) = 4.47 say, 4 accidents
Procedure to determine Crash reduction factor (CR)
 When multiple countermeasures are selected…
CR  CR1  (1  CR1 )CR2  (1  CR1 )(1  CR2 )CR3  ...
 (1  CR1 )...(1  CRm1 )CRm
CR = overall crash reduction factor for multiple mutually
exclusive improvements at a single site
CRi = crash reduction factor for a specific countermeasure i
m = number of countermeasures at the site
Example 5-6
CR1 = 0.40, CR2 = 0.28, and CR3 = 0.2. Determine the overall
CR factor. Note that countermeasures are ordered in the
descending order of their accident reduction factor values.
CR = 0.4 + (1 – 0.4)*0.28 + (1 – 0.4)(1 – 0.28)*0.2
= 0.66
Effectiveness of safety design features
(eventually we want to estimate the number of crashes that can be prevented (CP).)
In this chapter, we will see how (1) access control, (2)
alignment, (3) cross sections, (4) intersections, and (5) pedestrian
and bicyclist facilities might affect the overall safety of roadways.
Among these cross section related factors are used as an example
to compute CP values.
Access Control: Defined as “some combination of at-grade
intersections, business and private driveways, and median crossovers”
More access control  Less accidents
e.g. interstates
Streets
Access control (cont)
More access, higher
crash rates
Some methods to reduce
crashes by controlling
access:
 Remove access points
(remove median openings)
 Provide frontage roads for
business access
 Provide special turning
lanes (TWLTL or LT bays)
 Warn motorists of
changing conditions along
the roadway using proper
traffic control devices
(Note that the access control
section of the chapter does not
give CR values.)
Alignment (This topic was discussed in Ch. 16.
Review that chapter to find out what affected
vertical and horizontal alignment design.)
Vertical alignment  Most important factors include sight
distance (especially crest vertical curves) and the vertical curve
length.
Improvements to safety of horizontal curves include:
Improve the combination of
Use a less sharp H-curve
V- and H-curves
Widen lanes and shoulders
Assure adequate pavement
Add spiral transition curves
surface drainage
Increase the amount of
Provide increased skid
superelevation < max allowed
resistance
Increase the clear roadside
(Note that the alignment
recovery distance
section of the chapter does not
give CR values.)
Cross sections (this section gives CR values)
Clearance
(CR values)
Cross sections (cont)
(CR values) for shoulders
(Combined effects)
(AR values)
Cross sections (cont)
(CR values)
Table 5.11 is slightly different from other tables. It does not give CR
values. It gives % or cross-section related crashes (RC values) including
run-off-road, head-on, and opposite- and same-direction sideswipe.
Table 5.11 Ratio of Cross Section Related Crashes to Total Crashes on Two-Lane Rural Roads
Auxiliary lanes can reduce crashes (because
they provide safer passing opportunities.
F = fatal accidents
I = injury accidents
(CR values)
Example 5-7
Given:
Improvement options:
 A two-lane two-way highway
in mountainous terrain
 Widen 10-ft lane to 12-ft
lane (2 ft increase)
 53 crashes per year (3 year
average)
 Widen unpaved 2-ft
shoulder to paved 6-ft
shoulder
 Currently 10-ft wide lane, 2-ft
unpaved shoulder
 ADT = 4000 vpd
 A combination of the two
options
Find the expected number of accidents reduced:
RC = 53 x 0.61 = 32 related crashes (Tab 5.11)
a. Crashes prevented (CP) by lane widening = 32*0.23 = 7
accidents/yr (Tab 5.8)
b. CP by shoulder widening = 32*0.29 = 9 accidents/yr (Tab 5.9)
c.
CP by the combination = 32*0.46 = 15 accidents/yr (Tab 5.10)