Can the speed limit be used as a surrogate for the striking vehicle's travel speed or delta-V in police-reported rear-end crashes?

Objective: A recent study by Kidd (2022) recommended that organizations evaluating front crash prevention (FCP) systems like automatic emergency braking and forward collision warning increase speed differentials in existing test scenarios from 25 mph to 45 mph to make the tests more representative of police-reported rear-end crashes. Kidd used the posted speed limit as a proxy for the striking vehicle's travel speed prior to the crash. The current study used velocity data from event data recorders (EDRs) in rear-end crashes to evaluate this assumption. These same data were used to replicate another study (Farmer 2003) that showed the speed limit was a poor surrogate for delta-V in rear-end crashes. Method: A total of 11,199 crash records during 2017-2020 were extracted from the Crash Investigation Sampling System database. The analysis was restricted to 436 of these records that involved two vehicles with a front-to-rear manner of collision or rear-end crash configuration with EDR data from the striking vehicle. The relationships between the posted speed limit and striking-vehicle travel speed and between the speed limit and delta-V were modeled using regression. Results: On average, the speed limit overestimated striking-vehicle travel speed by 2 mph, but the relationship between the speed limit and travel speed was not linear. The speed limit reasonably approximated travel speed on roads with speed limits of 30 and S0 mph or higher. It slightly overestimated travel speed on roads with 40-45 mph speed limits and underestimated it on roads with limits of 25mph or less. The probability that the striking vehicle's travel speed was 25mph or less on any road was 0.09. In contrast, the probability of the striking vehicle's travel speed being 45mph or less was 0.54 overall and 0.75 or higher for roads with a speed limit between 25 and 45mph. As found in prior research, there was no significant relationship between the speed limit and delta-V. Conclusion: The posted speed limit was a reasonable surrogate for the striking vehicle's travel speed prior to police-reported rear-end crashes on roads with a speed limit above 25 mph. It was not a reasonable surrogate for delta-V. Travel speeds on roads with speed limits of 25 mph or less were much higher than the speed limit, which suggests that existing scenarios used to evaluate FCP system performance represent travel speeds in fewer police-reported rear-end crashes than previously thought. Increasing speed differentials in existing FCP test scenarios to 45 mph would reflect the striking vehicle's travel speed in three quarters of rear-end crashes on roads with a speed limit of 45mph or less.