PART A – Introduction, Human Factors, and Fundamentals
Part A describes the purpose and scope of the HSM, explaining the relationship of
the HSM to planning, design, operations, and maintenance activities. Chapter 3 in
Part A includes fundamentals of the processes and tools described in the HSM. This
chapter provides the background information needed to apply the predictive method,
crash modification factors, and evaluation methods provided in Parts B, C, and D
of the HSM. The chapters in Part A are:
- Chapter 1 – Introduction and Overview;
- Chapter 2 – Human Factors; and
- Chapter 3 – Fundamentals.
PART B – Roadway Safety Management Process
Part B presents suggested steps to monitor and reduce crash frequency and severity
on existing roadway networks. It includes methods useful for identifying sites with
potential for improvement, diagnosis, countermeasure selection, economic appraisal,
project prioritization, and effectiveness evaluation. The chapters are:
- Chapter 4 – Network Screening;
- Chapter 5 – Diagnosis;
- Chapter 6 – Select Countermeasures;
- Chapter 7 – Economic Appraisal;
- Chapter 8 – Prioritize Projects; and
- Chapter 9 – Safety Effectiveness Evaluation.
Chapter 4 of Part B includes several new network screening performance measures,
which shift the safety analysis focus away from traditional crash rates. Crash rate
analysis assumes there is a linear relationship between traffic volume and the frequency
of crashes. A focus on expected crash frequency can account for regression to the
mean when developing performance measures for network screening. Regression to the
mean is a statistical phenomenon where crash frequency naturally varies up and down
whether the location is treated or not. Using network screening methods that estimate
expected average crash frequency provides a more stable list of locations that might
respond to safety improvements than lists prepared with traditional methods. Part
B also provides methods for evaluating the effectiveness of an individual treatment,
a series of treatments, or an overall program, and for calculating a crash modification
factor (CMF). A CMF is a factor estimating the potential change in crash frequency
or crash severity due to installing a particular treatment.
PART C – Predictive Method
Part C provides a predictive method for estimating expected average crash frequency
at an individual site. The predictive method relies on safety performance functions
(SPF). SPFs are equations that estimate predicted average crash frequency as a function
of traffic volume and roadway characteristics (e.g., number of lanes, median type,
intersection control, number of approach legs). The chapters in this section are:
- Chapter 10 – Rural Two-Lane, Two-Way Roads;
- Chapter 11 – Rural Multilane Highways; and
- Chapter 12 – Urban and Suburban Arterials.
Predicting average crash frequency as a function of traffic volume and roadway characteristics
is a new approach that can be readily applied in a variety of ways, including design
projects, corridor planning studies, and smaller intersections studies.
PART D – Crash Modification Factors
CMFs quantify the change in average crash frequency as a result of geometric or
operational modifications to a site that differs from set base conditions. The various
chapters focus on specific treatments for the following:
- Chapter 13 – Roadway Segments;
- Chapter 14 – Intersections;
- Chapter 15 – Interchanges;
- Chapter 16 – Special Facilities; and
- Chapter 17 – Road Networks.
CMFs can be applied to any design or evaluation process where optional treatments
are being considered. The CMFs will also be a valuable addition to the documentation
of design exceptions.
