The initial phase of congestion affects road safety differently than the traffic jam itself; particularly the risk of serious crashes is higher during imminent congestion than in the traffic jam itself. When congestion starts to occur, driving speeds (gradually) decrease, vehicles decrease their following distance, and the smallest disruptions (for example, a driver taking his foot off the accelerator or gently braking) result in traffic (kinematic) shock waves. The speed of the traffic flow steadily decreases from the point where the disruption occurred, which coincides with frequent and hard braking. This increases the risk of rear-end crashes. In the traffic jam itself relatively little happens: traffic is almost stationary. If a crash occurs, speed is low and crash consequences are minor. However, there is a significant risk of crashing into slow moving or even stationary vehicles at the back of the queue.
From crash registration in BRON (the National police registration), the annual number of crashes in traffic jams or during the initial phase of congestion cannot be determined. In 2004-2009, however, these data were registered by the police and they show that about 8% to 12% of all crashes (so including those with property damage only) on national roads occurred during congestion. In previous and following periods, these data were not recorded or the registration rate dropped to 0%. Several older studies allow us to conclude that traffic jams lead to a decrease in injury severity, but to an increase in the number of crashes [5]. In line with this, recent SWOV research [6] shows that the risk of an incident-related crash [i] in free-flow conditions is five times lower than in congested conditions.
[i] as reported by the Rijkswaterstaat incident reporting system
