Empirical assessment of the impact of highway design exceptions on the frequency and severity of vehicle accidents

Compliance to standardized highway design criteria is considered essential to ensure the roadway safety. However, for a variety of reasons, situations arise where exceptions to standard-design criteria are requested and accepted after review. This research explores the impact that design exceptions have on the accident severity and accident frequency in Indiana. Data on accidents at roadway sites with and without design exceptions are used to estimate appropriate statistical models for the frequency and severity accidents at these sites using some of the most recent statistical advances with mixing distributions. The results of the modeling process show that presence of approved design exceptions has not had a statistically significant effect on the average frequency or severity of accidents -- suggesting that current procedures for granting design exceptions have been sufficiently rigorous to avoid adverse safety impacts.