| Advances in safety research---trying to improve the collective understanding of motor vehicle crash causation---rests upon the pursuit of numerous lines of inquiry. The research community has focused much on analytical methods development (negative binomial specifications, simultaneous equations, etc.), on better experimental designs (before-after studies, comparison sites, etc.), on improving exposure measures, and on model specification improvements (additive terms, non-linear relations, etc.).; One might think of different lines of inquiry in terms of 'low lying fruit'---areas of inquiry that might provide significant improvements in understanding crash causation. It is the contention of this research that omitted variable bias caused by the exclusion of important variables is an important line of inquiry in safety research. In particular, spatially related variables are often difficult to collect and omitted from crash models--but offer significant ability to better understand contributing factors to crashes.; This study includes a sizeable set of spatial variables such as chronic weather and environmental conditions, sun glare, proximity to drinking establishments, proximity to schools as well as demographic pattern along with traffic volume and geometric design elements to model signalized intersection crashes. Processing of the spatial data is done using Geographic Information System (GIS). For the purpose of modeling ordinary negative binomial (NB) and random effects negative binomial (RENB) models are used. Results indicate that spatial factors have strong influence in explaining total, pedestrian and bicycle crashes. Among the spatial variables sun glare has the highest impact in improving model explanatory power up to forty percent for total crashes. Spatial variables such as locations of colleges and universities, bars and pubs as well as demographic pattern near intersections are found to be very important in explaining total crashes. On the other hand location of elementary and middle schools as well as bars and pubs are very important in case of intersection related pedestrian and bike crashes. However, fatal and incapacitating injury crashes seem to be unaffected by the spatial variables. The research illuminates the importance of spatial variables in safety research and also the negative consequences of their omissions. |
