Negative effects of mid-block speed control devices and their importance in the overall impact of traffic calming on the environment

The purpose of this study is to investigate previously unknown effects of speed control devices on fundamental characteristics of traffic, such as vehicle headway distribution, absorption capacity and average delays to vehicles entering from driveways and pedestrian crossing opportunity. Headway data were collected at seven sites in the Sydney Metropolitan Area, under various levels of traffic flow and at various distances from the devices. The results demonstrate that although speed control devices do have some negative side-effects which are sometimes statistically significant, the magnitudes of the increase in average delays and the decrease in absorption capacities around the devices are below the level that would conceivably influence the practical crossing and merging abilities. These minor inconveniences to pedestrians and drivers are confined to the immediate vicinity of the devices and are, by far, outweighed by the benefits in terms of accident savings over the whole length of the street.