| The objective of this research was to compare safety, measured by expected crash frequency and severity, on road segments where design exceptions were approved and constructed to similar road segments where no design exceptions were approved or constructed. Data were collected for design exceptions in Utah in the years 2001 through 2006. Design exception request and approval forms, Google Earth, Google Street View, UDOT functional classification maps, and UDOT traffic volume data were used to identify and define road segments with and without design exceptions. Ultimately, a total of 48 segments with design exceptions and 132 segments without design exceptions were used for modeling. Propensity scores were applied in this study to assess the comparison sites (i.e., sites without design exceptions). The relationship between design exception presence and crash frequency was explored using a negative binomial regression modeling approach. The relationship between design exception presence and crash severity was explored in three ways: 1) computing severity distributions at locations with and without design exceptions, and 2) estimating separate negative binomial regression models by severity level. Design exception presence was represented in the regression models by an indicator variable (1 = one or more design exceptions; 0 = no design exceptions). Crash data from the years 2007 through 2010 were used for model estimation. Road segments with one or more design exceptions had the same expected frequencies of total crashes (all types and severities), fatal-plus-injury crashes, and property-damage-only crashes as road segments without design exceptions. |
