This paper analyzes the potential safety benefit from autonomous acceleration of an electrified lead vehicle to mitigate or prevent being struck from behind. Safety benefit was estimated based on the expected reduction in relative velocity at impact in combination with injury risk curves.
A possible way to do so is to make the lead vehicle speed up in order to reduce the relative speed at impact. To perform such an intervention however, quick response is required in the propulsion actuator which is why a traditional internal combustion engine (ICE) cannot be used whose response can vary significantly depending on the speed, transmission type, gear, turbo lag, etc. Electric drives, on the other hand, have very fast response in the order of tens of milliseconds [Hori et al. (1997)] and can be used to perform this intervention. Therefore, the lead vehicle is assumed to be electrified (fully electric or hybrid) in this paper in order to be able to perform the intervention.
In order to evaluate the potential of such an intervention to mitigate or avoid rear-end collisions, a hypothetical active safety system that uses acceleration on the lead vehicle is envisioned and used for analysis. This system, which works analogously to the AEB, is termed the Automatic Emergency Acceleration (AEA) system further on in this paper.
The lead vehicle is assumed to be able to reliably detect following vehicles behind it. This detection can be done either using sensors or using connected systems (vehicle-to-vehicle or vehicle-to-infrastructure) or by any other method. Due to the advent of advanced driver assistance and autonomous systems, such capabilities are likely to make their way into vehicles of the future.
In general, this study found that autonomously accelerating an electrified lead vehicle can mitigate and prevent rear-end collisions and significantly increase the safety benefits from existing systems such as autonomous emergency braking (AEB). While the AEB systems (already available on the market from several manufacturers) are estimated to be able to completely avoid 35 percent and mitigate 53 percent of all rear-end collisions [Schittenhelm (2013)], the remaining cases still account for a large number of accidents that can potentially be mitigated.
The AEA system alone can achieve velocity reductions of up to 15km/h when the following vehicle does not brake at all. When used in conjunction with AEB on the following vehicle, rear-end collisions with relative speeds up to 75km/h can be completely avoided which represents a 45km/h improvement over the outcome for the same case when AEB alone is used. These large velocity reductions achieved when the following vehicle brakes as well is in large part due to the fact that acceleration now not only reduces the relative speed, but also increases the distance available to the following vehicle for braking. Consequently, this benefit is obtained with relatively low increase in lead vehicle speed and distance travelled.
On average, a velocity increase and displacement of approximately 15km/h and 2-meters respectively can be seen. The peak speed increase is seen to be roughly 25km/h in order to achieve a velocity reduction of 75km/h. The corresponding peak displacement is approximately 5-meters which is roughly equivalent to the length of a large car.
Applying these findings to previous research the authors indicated that a reduction in change of velocity of 5km/h for the struck vehicle can decrease the risk of sustaining whiplash symptoms lasting for more than one month by up to 65 percent [Krafft et al. (2005)]. In terms of initial symptoms the same amount of reduction in change of velocity could decrease the risk by up to 40 percent [Krafft et al. (2005)].
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