AASHTO Journal, 24 October 2014
AASHTO has released a new two-chapter supplement to one of its most widely used publications for highway and safety engineering professionals, its Highway Safety Manual, First Edition that was issued in 2010.
In this new publication, AASHTO’s safety team provides predictive models to plan both freeway and interchange facilities, taking into account safety performance functions and crash modification factors. This is a targeted continuation of what the 17-chapter, 2010 manual provides in other areas – analytical tools to help transportation planners quantify and predict the likely effect of safety design decisions in all phases of highway project development.
Besides new chapters on “Predictive Method for Freeways” and “Predictive Method for Ramps” the supplement’s Appendix B contains specialized procedures common to both of those chapters.
The supplement’s Chapter 18 provides a methodology to estimate expected average crash frequencies for freeways with known characteristics and for all vehicle types. That information can apply to existing freeways and their design alternatives, or new freeways and alternative traffic volume projections. It also allows crash frequency estimates for past or future periods.
Chapter 19 allows modeling for ramps connecting two or more roads at an interchange, and for collector-distributor roads that connect with ramps and one or more other roads at an interchange. Like the prior chapter, it can be used for existing ramps or their design alternatives, for new ramps and for alternative traffic volume projections. Also like the freeways section, it allows for estimates of expected average crash frequency for either prior or future periods.
Appendix B explains two specialized procedures that are integral to the predictive models for both freeways and ramps. The first procedure is used to calibrate those models to local conditions. The second is the empirical Bayes Method, which combines observed crash frequencies with estimates generated by the models.
This Highway Safety Manual Supplement is available in print or as an online downloadable PDF, at a price of $175 for AASHTO member organizations, or $210 for non-members. Customers can place orders online here or by phone to the AASHTO Bookstore at 800-231-3475. This supplement’s item code is HSM-1S.