The Interactive Highway Safety Design Module (IHSDM) is a suite of software analysis tools used to evaluate safety and operational effects of geometric design on two-lane rural highways. IHSDM checks existing or proposed two-lane highway designs against relevant design policy values and provides estimates of a design's expected safety and operational performance. This paper describes efforts to apply IHSDM to two existing rural roadways (one an arterial and the other a collector) in Pennsylvania and to compare the crash prediction and speed profile results to observed speed and crash data for these same two road segments.
The crash prediction module results were evaluated based on historical crash data for the rural collector and arterial roadways. The use of site-specific crash history data, adjusted using the empirical Bayes (EB) procedure, and a statewide calibration factor, produced crash frequency predictions that closely matched observed crash frequencies. Observed operating speeds for both rural roadways were compared to the design consistency module outputs. For the rural collector roadway, speed profiles from a driving experiment provided the data to compare the design consistency module output. Additionally, data from the driving experiment were used to evaluate different desired speed inputs to the design consistency module. Point speeds observed at several locations along the rural arterial were used for the comparison to the design consistency module results.
The comparisons indicate that the recommended IHSDM desired speed input on high-speed roadways produces predicted speeds that match closely to observed speeds. On lower-speed roadways, the desired speed input should be based on field data.