Design Specific Safety Performance Functions and Crash Modification
by Søren Underlien Jensen and Winnie Hansen
The Danish Road Directorate commissioned Trafitec to develop tools that enable them to predict crashes, fatalities and injuries on a new or redesigned freeway or rural road, where the traffic regulation and road design have been specified.
Design specific safety performance functions (SPF) have been developed for the freeway and rural road network. The SPFs are negative binomial crash prediction models. However, a model is based on observations (links or intersections) of a highly specified design e.g. 4-armed roundabouts with one 6.5 m wide circulation lane, a central island of 30 m in diameter, aso. Crash modification factors (CMF) related to specified design elements, e.g. diameter of central island, have been recommended/estimated based on results from safety studies from around the world including new Danish studies.
Links, intersections and roundabouts were defined based on analyses. SPFs for the freeway network include models for exit ramps, entrance ramps, exit diverge areas, entrance merge areas and freeway links. SPFs for the rural road network include models for 4-armed roundabouts, 3- and 4-armed signalized intersections, 3- and 4-armed yield (give-way) intersections, and road links. Models for different severities of crashes and injuries have been estimated. Around 90 CMFs are recommended/estimated.
All SPFs and CMFs have been inserted into spreadsheets, which enable traffic planners to easily calculate numbers of crashes, fatalities and injuries on a new/redesigned freeway, rural road, ramp, intersection or roundabout.