Investigation of potential disbenefit scenarios with bicycle-detecting automatic emergency braking systems

OBJECTIVE: Bicycle-detecting automatic emergency braking (B-AEB) systems have the potential to prevent and mitigate many vehicle-bicycle crashes. Vehicle deceleration from B-AEB could result in a potential disbenefit scenario by changing the impact configuration from the bicycle striking the vehicle side to the bicycle being struck by the vehicle front. The purpose of this research was to use simulation to investigate the potential disbenefit of B-AEB in straight crossing path scenarios. METHODS: This study used an analytical set of equations to determine the collision configuration between a small car and bicycle based on speed and the time-to-intersect (TTI) of the straight crossing paths. The vehicle speed was varied from 3.6 to 130 km/h, and the bicycle speed was varied from 3.6 to 28.8 km/h. Starting positions were varied from 0.1 to 3.2 s before the vehicle and bicycle paths intersected. B-AEB deceleration magnitudes from 0 g to 0.9 g were simulated. In total, this study simulated over 127 million scenarios. The potential disbenefit scenario in simulation was compared with real-world bicycle crash data. RESULTS: The configuration in which the bicycle struck the vehicle side occurred in a larger proportion of crash scenarios at lower vehicle speeds. For example, a bicycle traveling 15 km/h struck a vehicle traveling 40 km/h in 31.2% of crash scenarios but only struck a vehicle traveling 80 km/h in 18.2% of crash scenarios. At the same time, B-AEB systems avoided more crashes when the vehicle was going slower. At a vehicle speed of 40 km/h, B-AEB systems that activated at least 0.75 s TTI with over 0.7 g of deceleration were able to completely avoid the potential disbenefit scenario. Simulation results indicated that two existing B-AEB systems, one that braked at 1.05 s TTI at 0.9 g and the other at 1.2 s TTI at 0.7 g in controlled testing, would avoid these disbenefit collisions up to vehicle speeds of 60 km/h. At vehicle speeds over 60 km/h and activations over 0.75 s TTI, B-AEB could change the configuration but the impact speed would be reduced to below 37 km/h, which is associated with a less than 5% risk of serious injury for the bicyclist. CONCLUSIONS: This study considered all straight crossing path crashes between a small car and a bicycle and considered the circumstances under which B-AEB could lead to a change in crash configuration resulting in a potentially worse outcome. Among the crash scenarios with a potential for disbenefit with B-AEB activation, current B-AEB systems are activating early enough and braking hard enough to simply avoid the collision at speeds up to 60 km/h and mitigate the probability of serious injury at higher speeds.