  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.
