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Designing Roads That Guide Drivers To Choose Safer Speeds

This report describes an investigation into whether or not physical characteristics of the roadway and the roadside environment are associated with actual vehicle running speeds, and how actual vehicle running speeds are associated with the occurrence and severity of motor vehicle crashes in conjunction with other roadway and roadside characteristics. Actual vehicle running speeds were observed at about 300 locations in urban, suburban and rural areas across Connecticut, at locations without horizontal curves or traffic control devices. Only vehicles traveling through the section unimpeded either by leading or turning vehicles were observed in order to get true free flow traffic speeds. Roadway and roadside characteristics were observed, and statistical prediction models were estimated to learn more about how free flow vehicle speed, roadway and roadside characteristics and crash incidence and severity are related. The factors associated with higher average running speeds are wide shoulders, large building setbacks and a residential location.

The factors associated with lower average running speeds are on-street parking, sidewalks and a downtown or commercial location. These findings suggest that drivers slow down where the road feels “hemmed-in” or there is noticeable street activity, and they speed up where the road feels “wide open” or street activity is less noticeable. This finding is not surprising, but these relationships are quite strong in the observed data, and it is a useful result to isolate this short list of factors that are significantly correlated with actual vehicle running speeds.

These findings demonstrate that through careful, intentional selection of roadway and roadside design elements, it is possible to influence the running speed of traffic on a road. It appears that drivers indeed take cues from elements of the roadway and roadside environment to decide how fast to drive and these cues are independent of the posted speed limit and other considerations that might be important to the community for reducing speeds. So the good news is that it is possible to influence drivers’ choice of speed through design of roadway and roadside elements; but the bad news is that many existing roads cue drivers to travel much faster than the posted speed limit and the community would like.