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FOREST SERVICE HANDBOOK
Portland, Oregon
TITLE 7709.59 - TRANSPORTATION SYSTEM OPERATIONS HANDBOOK
R-6 Supplement No. 7709.59-92-1
Effective May 6, 1992
POSTING NOTICE. Supplements to this handbook are numbered consecutively.
Check the last transmittal sheet received for this handbook to see that the above
supplement number is in sequence. If not, obtain intervening supplement(s) at once
from the Information Center. Do not post this supplement until the missing one(s)
is received and posted. After posting, place the transmittal at the front of the title
and retain until the first transmittal of the next calendar year is received.
This is the first R-6 Supplement to this handbook.
Document Name
50
Superseded New
(Number of Sheets)
6
Digest:
51.4 - Provides additional direction to achieve better consistency Regionwide in the
identification and mitigation of factors which affect both the probability and
severity of accidents.
/s/ John F. Butruille
JOHN F. BUTRUILLE
Regional Forester
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7709.59 - TRANSPORTATION SYSTEM OPERATIONS HANDBOOK
R-6 SUPPLEMENT 7709.59-92-1
EFFECTIVE 5/6/92
CHAPTER 50 - TRAFFIC STUDIES AND OTHER ROAD OPERATION
CONSIDERATIONS
51 - Traffic Studies.
51.4 - Hazard Analysis.
When reviewing a site or segment of road to identify required hazard mitigation,
first determine the probability of an accident occurring. Next estimate the severity
of potential accidents. The combination of probability and severity is a measure of
the risk of accidents at the site or segment.
The hazard analysis should include anticipated use and a review of accident
records. The completed analysis should be periodically reviewed to determine if
some change in conditions occurs which changes the results of the analysis.
1. Probability of Accidents. Probability deals with forecasting the effect of
factors present in any situation and the likelihood of an accident resulting from
exposure to those factors. Consider the following factors in forecasting effects:
a. Accident History - Determine if there has been a history of near misses or
accidents. Review all available historical accident records. Field evidence of skid
marks, chrome strips, glass, or marks on existing features indicates problems.
When long term, reliable information concerning the frequency of accidents at a site
is available, that information should heavily influence estimates of future
occurrences.
b. Traffic Volume and Type - A factor which can have a large influence on
probability. Consideration should include total traffic volume, traffic composition,
and driver characteristics. Driver characteristics include familiarity with Forest
Service roads and ability to recognize and navigate the particular situation
involved.
c. User Speed - The speed at which users normally travel the roadway. (This
may be considerably greater than the speed for which the roadway was designed.)
The probability of accidents increases significantly if users consistently exceed the
safe speed for a given portion of road and thus reach a hazard before they are able
to identify and respond to it.
d. Roadway Cross Section - Changes in the physical features of the cross
section including template, slope, shoulder width, or surface type increase
probability.
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e. Visibility - The opportunity for users to observe and recognize a hazard or
hazardous situation will affect their ability to take appropriate action. This
includes approach alignment, fog, roadside vegetation, dust problems, combined
horizontal and vertical curves, and other conditions which reduce advance warning
of a hazard.
f. Roadway Alignment - The outside edges of horizontal curves are more
likely to be a problem than those on the inside edge, especially if the driver is not
anticipating the amount of speed reduction required. Sharp vertical curves may
affect visibility and control. Steeper grades coupled with horizontal curves which
direct the vehicle toward a hazard will increase probability for accidents.
g. Climatic and Surface Conditions - Unexpected roadway surface conditions
at site such as snow patches, black ice, slick pavement, washboards, pot holes, or
loose gravel will increase the probability of accidents. Consideration must be given
to roads intended for use during times of year when the roadway is covered with
snow or ice.
Low Probability exists where there is a combination of factors such as:
* no known accidents
* lower ADT (30 or less)
* users are well acquainted with the situation
* lower speeds of 15 mph or less generally associated with maintenance level
2 roads
* abrupt changes in roadway cross section are not present
* little or no impairment of visibility
* changes in roadway curvature are smooth and do not require rapid
deceleration
High Probability exists where there is a combination of factors such as:
* history of several accidents
* ADT in excess of 150
* users are present who may not be familiar with this type of road or driving
* speeds in excess of 40 mph generally found on maintenance level 4 and 5
roads
* changes in road width, shoulder or surface type occur
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* some abruptness of either vertical or horizontal curvature are present and
changes in speed and maneuvering are required
2. Severity of Accidents. Severity relates to the probable result of an
accident and can range from minor (fender bender) to high (critical injury or loss of
life). The following factors should be considered in predicting the severity of
accidents:
a. Height of Fill and Steepness Below Roadway - The slope and the height of
constructed embankments and natural ground slopes below the roadway have a
direct relationship to the probability of major damage or serious injury/death as a
result of "ran-off the road" accidents.
b. Roadside Conditions - Large, unyielding features adjacent to the road
(trees, bridge abutments, boulders, etc.) increase the potential severity as compared
to those that are set back from the road or are more yielding. This severity may be
increased where man-made features exist which are not in conformance with the
contour of the natural setting.
c. When lakes with sufficient water depth to immerse vehicles or fast moving
streams are adjacent to the roadway, consideration must be given to possible
drowning of vehicle occupants in ran-off the road accidents.
d. User Speed - Probably the single most important factor to be considered.
While it may be modified by other factors, in general, the higher the speed, the
higher the severity.
e. Traffic Type - Accidents involving multi-passenger vehicles could be more
severe than accidents involving single-passenger vehicles.
f. Climatic Conditions - Travelway surface with conditions such as snow, ice
or fog can be a major contributor to an increase in severity when placed in
combination with one of the other factors.
Low Severity - A combination of factors such as:
* slower speeds (15 mph or less)
* adequate clearance from hazards, limited obstacles, and shallow streams or
other bodies of water.
* lessor slope steepness
* fair alignment and visibility
* single or family passenger vehicles
* Travelway is relatively clear of fog, snow, or ice
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High Severity - A combination of factors such as:
* higher speeds (40 mph or more)
* little clearance for roadside hazards, intrusions in roadway, deep, or fast
water
* steep grades (over 12%)
* steep side slopes or drop-offs
* radical change in user speed or alignment
* buses or other similar multiple passenger vehicle
* fog, snow, or ice are common during use
After completion of the ranking for probability and severity, determine the most
cost-effective method of managing the accident risk. Reduction of risk needs to be
balanced against the investment required to reduce the risk. At some locations the
cost to eliminate most or all accidents may not be cost effective. A less costly
treatment which allows a moderate accident frequency may be the most costeffective solution if the accident severity can be reduced to a minor accident risk for
the average driver.
The use of guardrails is one of several methods available to mitigate hazards.
Guardrails may be suitable on low-volume roads where they are the only practical
and feasible means of mitigating road inconsistency and reducing accident severity.
Other methods such as delineators, signing, berms, false cuts, insloping, or varying
alignment may be an appropriate and less costly means for reducing hazards to the
road user. All alternatives should be considered at each location for their
effectiveness.
In addition, treatments must consider the Road Management Objectives for
Recreation Opportunity Spectrum (ROS) and Visual Quality Objective (VQO).
Guardrails are in conflict with desired ROS settings of "semi-primitive" and "roaded
modified" and may be inappropriate in the "roaded natural" setting. Certain
roadside treatments may conflict with a retention VQO.
Treatments to reduce risk not only vary in cost and sensitivity to management
objectives but also in the likelihood of the treatments being in place when they are
needed. At sites which have high risk treatments, guardrail and berms should be
considered as a starting point. Other treatments (signing, speed bumps, alignment
changes) may be suitable at these high risk sites if they can dependably manage the
risk. These "other" treatments should be considered as the starting point for lower
risk sites.
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The following graph shows the spread of treatments for the various rankings of
frequency and severity.
High
(PROBABILITY)
Low
|
|
|
|
|
|
|
| A
|
Low (SEVERITY)
B
High
The treatments for sites in the A position on the graph should be none or very minor
while the treatments for those sites in the B positions of the graph should be more
extensive and permanent.
When analyzing existing or proposed bridge sites, also refer to 7709.56b, Drainage
Structures Handbook, Subsections 7.34 - Bridge Railings and 7.35 - Approach
Railing.