Free-flow vs car-following speeds: does the difference matter?

Speed is one of the fundamental characteristics of traffic flow and also one of the most influential risk factors – it is therefore important from the perspective of safety-related studies. For these purposes speed data are often collected by stationary radars. However, since free-flow speed is usually needed, the user has to distinguish between free-flow and congested conditions in the collected data. Although this problem has long been acknowledged, there is no single and simple approach towards it; various authors have used different threshold definitions and values. In this regard, the authors aimed to investigate the amount of difference between free-flow and congested speeds, using a series of stationary radar speed surveys on Czech rural roads. It was found that difference (in terms of vehicle gaps) between free flow speed and car-following speeds is of concern; in addition, the magnitude of these differences is not constant as suggested in previous studies. It was also shown that there is a positive relationship between the gaps and accident rates in the studied road sections. These findings should be considered in future analyses which aim to obtain unbiased results of speed and its derivatives such as a proportion of speeding for safety-related analyses.