Distance-rela­ted beha­viour on motor­ways

The failure to maintain adequate safety distance is the cause of many accidents on motorways. In 2022 17,755 accidents involving personal injury on motorways and 23,190 associated driving errors were recorded. Nearly a third of those errors document insufficient safety distance.

In the ten years prior to the COVID-pandemic (2009 to 2019) the proportion of accidents caused by insufficient safety distance (accident cause 14) on motorways rose from around 20 to 30 percent. In 2022 23,190 driving errors were recorded in the context of accidents on motorways. 6,791 of those, and by that around 30 percent of all officially recorded driving errors, were classified as “insufficient safety distance”. Against this backdrop the German Insurers Accident Research (UDV) has conducted a research project in cooperation with the Chair of Road Planning and Road Design at Dresden University of Technology. The purpose of the project was to analyse and provide a scientific description of the distance behaviour on motorways with the help of drone measurements and to derive recommendations for improving road traffic safety on these motorways.

The accident analyses carried out in the context of the project show, that “insufficient safety distance” is the most recorded driving error of accidents involving personal injury on motorways. More than 90 percent of those are accidents in longitudinal traffic and more than 80 percent are collisions with a vehicle moving ahead or waiting. The accidents are often caused by car drivers, with increasing injury severity the proportion of accidents caused by truck drivers is increasing though. An analysed sample of around 300 descriptions of accidents with cause 14 shows that those accidents often happen in the context of increased traffic volume or traffic disruptions (e.g. traffic jams).

At 25 measuring points (20 on the open road and five at junctions) the distance and speed of vehicles as well as their distance behaviour when changing lanes were recorded with the use of drones and analysed. The findings can be summarised as follows:
 

1. The more traffic drives on the motorway, the more often short distances of less than two seconds between to vehicles occur.
2. Insufficient distances are more frequent on the left-hand lane than on the right-hand one.
3. Car drives maintain short distances more often than truck drivers.
4. With cars the proportion of short distances to the vehicle in front increases linearly with increasing utilization of the road capacity. This is not the observable with trucks.
5. Concrete reasons for the occurrence of accident clusters with the cause “insufficient safety distance” could not be derived from the analysed behaviour as no differences in the distance and speed behaviour at accident clusters and checkpoints could be identified.
6. Insufficient distances are more frequent on on-ramps than on off-ramps. At the same time driving speeds are marginally lower on on-ramps than on off-ramps.
7. Lane changes to the left more often lead to critical situations because of frequently occurring short distances between vehicles and the need for the trailing vehicle to brake.
8. When changing from the right-hand lane to the left-hand lane and when entering the motorway from the on-ramp gaps of less then 100 meters are used most often.
9. A direct influence of lane changes on the occurrence of accidents could not be determined on the basis of the accident analyses carried out.

Based on the findings and conclusions from the analyses carried out on accidents, distance behaviour and lane changes, the following recommendations can be derived.

Infrastructure and traffic
 

• On accident-prone stretches of motorways with regularly occurring traffic jams, the traffic flow should be harmonized by means of traffic management or network management systems so that traffic jams are best avoided and safe distances between vehicles maintained. The measures can also include temporary speed restrictions or no-overtaking zones. It may be necessary to enforce these measures by means of effective fixed or mobile speed and distance monitoring systems.

Networking of infrastructure and vehicles

• The use of traffic jam warnings to allow drivers to adapt their behaviour (e.g. their speed and distance) as well as real-time warnings of the ends of traffic jams at navigation devices installed in vehicles and at mobile devices from other mobility service providers can help prevent or reduce accidents resulting from insufficient safety distances on motorways.
• The networking of infrastructure and vehicles using cooperative intelligent traffic systems (C ITS) should be encouraged to support road users in adapting their driving behaviour in good time.

Advanced driver assistance systems

• The mandatory installation of advanced driver assistance systems (ADAS) in new vehicles pursuant to the European Parliament’s General Safety Regulation (GSRII), for example in the form of automatic emergency braking systems, emergency stop signals, drowsiness and attention warnings and intelligent speed adaptation (ISA), will have positive effects on road traffic safety. However, their effectiveness in practical use should be systematically analysed in order to permit changes to their design as required.
• The more widespread diffusion of level 2 assisted driving functions for cars and trucks may help harmonize the traffic flow on motorways and possibly also lead to improvements in road traffic safety. These functions consist of intelligent ACC (adaptive cruise control) that adapts both the safety distance to the vehicle in front and the driving speed to the speed limit and also keeps the vehicle centred in the lane. These are level 2 assisted driving functions which continue to leave the driver with full control of driving tasks. EuroNCAP assesses these systems because correctly conceived driver involvement in driving tasks is of vital importance for road traffic safety. This applies equally to cars and trucks. A further development to level 3 driving functions, in which the driver hands over the driving tasks to the vehicle, could have a further positive effect on road traffic safety. The effectiveness of these systems should also be analysed in the context of real-life accident occurrence in order to identify any negative effects at an early stage.
   

Traffic behaviour and campaigns

• Appropriate campaigns might further raise road users’ awareness of potential dangers arising from disruptions to the traffic flow.
• Raising awareness among car drivers of the distances to be maintained through campaigns that explain the applicable regulations in easy-to-understand terms based on the schedule of penalties for road traffic offences (Annex, Table 2).The campaigns should be accompanied by reinforced, regular mobile and, if appropriate, fixed distance and speed monitoring. This also applies in case of dynamic speed limits made necessary in the light of traffic conditions.
   

Outlook
 

• The attribute “Traffic jam” should be included in road accident reports as a specific feature of motorway accidents.
• Further-reaching research on the influence of visibility on safety distances as well as on accidents and their consequences may provide further insight that can help prevent or reduce accidents due to insufficient safety distances on motorways. This should be complemented by research on the influence of lane changes (on open stretches of road as well as at junctions) on accident occurrence and traffic flow on motorways.