Improving speed limit compliance in work zones has been the focus of a number of safety-related studies. Speeding is a particular concern under non-congested traffic conditions when motorists can freely select their speed without being constrained by other vehicles. The effectiveness of a particular speed control really depends on how it influences the behavior of the fastest drivers, particularly those whose speed selection is unaffected by other vehicles around them. A common rule of thumb to separate the so-called "free-flowing" vehicles from a traffic stream is to set a time headway threshold (e.g., 5 seconds), and consider only those vehicles which have headways greater than the preset threshold. Although this time headway criterion has been widely accepted, there are some problems with its use. In certain cases, motorists keeping longer headways may still be affected, while others keeping shorter headways may be unaffected by the vehicle in front of them. This paper identifies potential problems associated with the use of constant headway thresholds, and recommends a solution to overcome these problems. A methodology using variable headway thresholds is presented, and its application is illustrated using speed and headway data collected in a freeway work zone in Texas. The advantage of using the proposed variable headway thresholds is that they implicitly account for differences in a combination factors. They are determined from raw data collected at each site, and provide a more realistic threshold than the constant headway criteria for separating free-flowing and constrained vehicles for the purpose of evaluating speed control measures. Therefore, they are expected to improve the reliability of evaluations of speed control devices and strategies.
Samenvatting