I would suggest that speed (or better defined as speed differential) is the prime factor in nearly all road traffic accidents. Surely, he is not suggesting that there was no measurable momentum involved in the 81 accidents to which he refers.
Two unusual examples illustrate this point:
A tree branch breaks off in a strong wind and falls on a parked vehicle. The wind speed is a factor in the branch breaking and the speed caused by gravity is a factor in the impact.
Another example is from experience: speeding from my shower to answer my telephone resulting in a sliding fall causing bruising and soreness.
All such calls are now left to my machine.
Relative velocity is a factor in most road accidents. The seriousness of such accidents is closely related to the speed differential of the vehicle or vehicles involved and always has been e.g. car/tree, car/bicycle, head-on where the differential can be more than 200km/h and many others; even falling off your bicycle or horse.
Velocity differential is the most likely main determiner of the seriousness of resulting injury and property damage.