Integrated Pedestrian Safety Assessment: A Method to Evaluate Combinations of Active and Passive Safety

Doctoral thesis, 2015

Pedestrian road casualties are a major concern in many countries. Vehicle safety systems attempt to reduce casualties, and the accurate assessment of such systems is therefore essential. Passive safety assessment is well established, and additional active safety assessment has recently emerged. However, assessment methods accounting for the interaction between active and passive safety do not exist in today’s regulatory or consumer testing. An integrated safety assessment can help reduce pedestrian casualties more effectively and efficiently by taking information gained through active safety assessment into consideration and modifying the passive safety assessment accordingly. This research develops an integrated pedestrian safety assessment method and demonstrates its use in assessing combinations of passive safety and the active systems of Automated Emergency Braking (AEB) and Forward Collision Warning (FCW). Firstly, a method was developed that predicts causality costs for a vehicle using data from passive safety and AEB evaluations. Casualty costs were then compared for vehicles with good, average or poor Euro NCAP passive safety ratings in combination with an A-pillar airbag and an AEB system. The results show that the AEB system has a safety benefit broadly equivalent to increasing the Euro NCAP passive safety rating from poor to average or average to good, and that the estimated benefit of the A-pillar airbag exceeded that of the AEB system. Secondly, the method was extended to assess FCW systems. Data to model driver reactions required for the FCW assessment were obtained in a volunteer study. Applying this method for different types of FCW systems showed that such systems can, but do not necessarily, provide benefits similar to those of AEB systems. An early activating FCW system with a haptic (brake pulse) warning interface was as effective as an AEB system in reducing casualty cost. These assessments of AEB and FCW systems measure True Positive performance, which is, broadly speaking, the performance of an activated system in situations in which activation was needed. Additional False Positive requirements are proposed to ensure that active safety systems are not activated too early; a threshold of what could be considered too early was developed from the quantification of driver comfort boundaries in volunteer studies. The integrated assessment method proposed has the benefit of estimating overall safety performance with a single indicator, casualty cost, making results for different vehicles easily comparable. Furthermore, as the method aims at a realistic assessment of a vehicle’s ability to protect pedestrians, all body regions and injury severities, as well as impact kinematics, all relevant impact speeds, and their interdependencies are taken into account making this the most complete method currently developed. However, since the method relies on the testing of a vehicle’s active safety systems in representative scenarios, and on the testing of its passive safety with existing impactor tests, limitations of these existing test procedures will necessarily have an impact. It is suggested that the proposed integrated pedestrian safety method be implemented in consumer testing to assess the total benefit offered by any combination of active and passive safety technology. In addition, findings suggest that testing for active safety should be expanded to FCW systems and, furthermore, that False Positive tests should be implemented. In the test scenarios already in use for assessment of speed reductions, AEB and FCW system activation before comfort boundary timing should be discouraged. On implementing these proposals, assessment would more accurately reflect the total safety benefit offered by different systems and therefore aid the development and proliferation of the most effective and efficient pedestrian safety systems.