UN Regulation No. 137 (R137) specifies a 50 km/h, full-width rigid barrier frontal impact test with driver and passenger 50th and 5th percentile Hybrid III dummies respectively. One objective of the regulation is to encourage better restraint systems that protect older occupants and across a wider range of collision severities.

This paper investigates two research questions:​

• How much will restraint systems of European cars have to be modified to meet the requirements in R137?
• What level of protection is offered by current European restraint systems at lower impact energies than tested in R137?

Six full-scale crash tests were conducted with European-specification supermini cars. The test configurations used were R137 in standard configuration, at reduced impact speed and with a THOR-M ATD instead of Hybrid III. The crash tests were complemented by an analysis of Road Accident In-Depth Studies (RAIDS) data.

The test vehicles in their European market specification were found to already meet the minimum performance requirements set out in the future R137 at 50 km/h for Hybrid III ATDs. The THOR-M ATD generally predicted greater injury risk than the Hybrid III and in some conditions exceeded the current regulatory limit values (as defined for Hybrid III). At lower impact speeds of 35 km/h, the key driver thorax injury metric measured with THOR-M was found to be only slightly reduced.

The accident data analysis showed that a considerable proportion of casualties sustaining MAIS 2+ or MAIS 3+ injuries occur at impact energies which are lower than currently proposed in R137. It was also found that the thorax was the body region most prone to AIS 2+ or AIS 3+ injuries in low-energy impacts. Older occupants (66 years and older) were markedly over-represented in the low-energy casualty groups.

Under the conditions set within R137, it is likely that many European vehicles will pass without requiring significant changes. Therefore, in its current state, there is no evidence that R137 encourages better driver restraint performance. The results at reduced impact test speeds further indicate that occupants could be more vulnerable than necessary at lower collision speeds.

The accident data further show that there might be a large target population for a low-energy restraint test. The composition of this casualty group indicates that the force limits of current seat belt load limiters might be too high for the reduced biomechanical tolerance of elderly occupants and higher than necessary in low- speed collisions. The accident research is based on UK accident data, which means that the sample size was limited and the results may not be representative of other countries. However, the general trends identified align with previous data from other European countries.

It was concluded that implementation of the THOR-M ATD as a replacement for the current Hybrid III in R137 should be considered at the earliest opportunity in order to deliver tangible benefits. Test and performance requirements could be set to encourage adaptive restraints which provide better protection at lower impact energies.