A MODEL FOR AVERAGE SPEED ESTIMATION AND CRASH PREDICTION USING VEHICLE PATH DATA

Research has shown roundabouts to significantly lower expected crash rates compared to traditional intersection designs. This improvement in safety is believed to be related to lower vehicle speeds and the more favourable angles between conflicting vehicle paths which lead to less severe crashes. Since modern roundabouts are designed to slow down traffic, speed control is essential to optimizing safety performance. It also stands to reason that the expected safety performance of a roundabout can be related to expected vehicle speeds. Safety Performance Functions (SPFs) are statistically derived equations that predict the expected crash frequency of a site. SPFs have been developed both in the U.S. and internationally for roundabouts. However, the SPFs developed tended to include few variables useful for evaluating the safety impacts of alternate designs. This is perhaps not surprising given that such functions are in fact difficult to estimate, given that roundabouts tend to have very few crashes and design features with little variation. More recent research has modeled crashes at roundabouts as a function of vehicle speeds at various points in the roundabout. The premise being that if the safety performance of a roundabout can be related to vehicle speeds, and speed can be better estimated than crash experience can, then speed can be used indirectly as a surrogate in evaluating the safety implications of decisions in designing or re-designing a roundabout. The database of roundabouts used for the current research included geometric, traffic, speed and crash data for 34 roundabout approaches. The speed data included the average vehicle speed for approaching, entry, upstream circulating and upstream exiting vehicles. The database was supplemented with the entry, circulating and exiting vehicle path radii for the design vehicle, the entry angle and the predicted vehicle speeds obtained from the TORUS Roundabout software. In the present research, average vehicle speeds were modeled using the vehicle path radii predicted using TORUS Roundabout. It was found that the model predicting the average of entering, exiting and circulating speeds using vehicle path radii from TORUS Roundabout performs well. These predicted speeds could in turn be used with existing crash prediction models which use vehicle speeds to predict crashes.