Flagger Effects in Reducing Likelihood of Rear-End Collisions in Work Zones

Rear-end collisions are among the most dominant crash types in work zones. This study explores the effectiveness of flaggers in reducing the frequency of very short headways and the likelihood of rear-end collisions in work zones. To assess the effectiveness of flaggers, eleven data sets were collected from five single-lane work zones with no lane change opportunity. Time gap and speed data of more than 6,250 vehicles were used. Statistical analyses were made based on the frequency of three risky driving behaviors that may lead to rear-end collisions: I) Maintaining “short” time gaps (i.e. following “too closely”), II) Speeding by at least 5 mph at “short” time gaps, III) Faster-trailing by at least 5 mph at “short” gaps. Based on the distribution of driver reaction times, three different upper thresholds were used to define “short” time gaps: 0.70 s, 0.85 s, and 1.00 s.

The results indicate that flagger presence significantly reduced the probability of a vehicle’s following “too closely” by around 60% to 69%. Likewise, it led to an 85% to 95% reduction in the probability of a vehicle’s speeding by at least 5 mph at “short” time gaps. Notwithstanding these, the frequency of faster-trailing by at least 5 mph at “short” time gaps was found to be trivial whether there was flagger or not. Yet the flagger presence was a key factor that significantly reduced the likelihood of rear-end collisions at the study sites.