Safety Impacts of Design Exceptions

Compliance to the Indiana Department of Transportation's (INDOT) 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 36 roadway sites with design exceptions and 71 without design exceptions are used in this research, and appropriate statistical models are estimated for the severity and frequency of these accidents. The results of the detailed statistical modeling show that presence of design exceptions, approved by INDOT, do not have a statistically significant adverse effect on the frequency or severity of accidents. While the data are too limited to investigate the effect of specific design exceptions (the number of Level One design exceptions granted is a modest number), the research herein shows that INDOT procedures for granting design exceptions have been sufficiently strict to avoid adverse safety consequences and that current practices should be continued. To guide future Level One design exceptions, the detailed statistical findings of this research effort suggest that using previous design exceptions as “precedents” would be the best way to proceed. To this end, it is recommended that INDOT maintain a database of Level One design exceptions.