Variable time headway criteria to improve reliability of speed control system evaluations
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The effectiveness of a particular speed control system depends on how it influences the behavior of those drivers whose speed selection is unaffected by other vehicles around them. A common rule of thumb that can be used to separate these so-called unconstrained or free-flowing vehicles from a traffic stream is to set a time headway threshold (e.g., 5 s) and consider only those vehicles that have headways greater than or equal to 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, whereas 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 an alternative approach. A methodology that uses variable headway thresholds is presented, and its application is illustrated with speed and headway data collected in a freeway work zone in Texas. The advantage of the proposed variable headway thresholds is that they can account for a number of site-specific variables, such as roadway geometry and traffic and driver characteristics. They are determined from the actual raw data collected at each site and therefore provide a headway threshold more realistic than the constant headway criteria used to separate free-flowing and constrained vehicles in evaluations of speed control measures. Therefore, in many cases, they are expected to improve the reliability of evaluations of speed control systems.
