WE NEED A VEHICLE WEIGHT TAX for excessively heavy vehicles (possibly excluding freight since truck drivers get screwed so hard these days anyway). The extra wear on the roads is reason enough to massively tax >2 ton vehicles.

Smaller vehicles tend to be more nimble, thus increasing accident avoidance.

Fatalities per vehicle mile sounds scientific but congestion (vehicles per lane miles) is a major factor.

Re vehicle momentum: I’d think that the mass of driver and passenger is more pertinent than vehicle mass except in high/low mass collisions.

I remember the same argument back in the 70’s yes, I’m that old) when Japanese “econobox” vehicles started sharing the roads with hulking steel American cars that got 7mpg. If there was an uptick in automobile deaths, it must have been short-lived and minor as I have no recollection of it. “Safety standards” and “crumple zones” made vehicle accidents more survivable.

As people demanded more fuel-efficient cars, the average weight of vehicles on the road began to decline. It has only been the recent “SUV craze” that has loaded our highways with bulky gas guzzlers again. And just as it did in the 70’s, increased fuel efficiency will drive down the average weight and increased safety standards will decrease casualties.

In a collision between a large and a small vehicle the small vehicle will tend to take the brunt of the damage. The larger vehicle has more momentum to dissipate in the accident. However, looking at only this one scenario is misleading.

Several other aspects bear consideration.

First, larger vehicles are more susceptible to single vehicle accidents. Large trucks and SUVs have more momentum and that can lead to stability problems like we’ve seen in Ford Explorers. These vehicles are more prone to rolling and to damage from collisions with road side objects.

Second, the passengers within a larger vehicle may be in more danger than those in a smaller vehicle. The momentum of the crash may be transferred to the occupants of the vehicles. The occupants of the higher momentum vehicle will tend to be injured in greater proportion, particularly if they are not wearing seat belts or if airbags do not deploy properly.

Finally, although a collision between a larger vehicle and a smaller vehicle endangers those in the smaller vehicle, in collisions between like vehicles those in the smaller vehicles will tend to be safer. At the same speed, a collision between smaller vehicles must dissipate much less momentum than a similar collision between larger vehicles. A road which contains a greater proportion of smaller vehicles may be safer overall to the drivers of those vehicles, and even to the drivers of the larger vehicles.

Beyond momentum arguments, the safety features of cars have greatly improved since the seventies and the eighties. Modern cars have rigid passenger compartments surrounded by crumple zones, airbags often for the rear passengers as well as for the driver, as well as improved seat belts, anti-lock brakes, and traction control systems. I don’t think you can look at studies from the seventies and eighties and then deny all the advances which have happened since.

But then, I don’t think Grover Norquist is actually interested in having an honest debate about these issues. He’s interested in scaring people so his organization can raise more money.