Now that I tried it out, I'd like to offer further feedback regarding the weight class.
In the encounter that we've played there were 5 motorcycles and two cars. Having rolled the dice, their assigned weight classes were:
2, 5, 8, 12, 12 for motorcycles (light weight, 1d12)
4, 7 for cars (medium weight, 2d6)
The situation where light vehicles were assigned higher weight class than medium vehicles was, in fact, quite likely to occur. As presented on https://anydice.com/program/877 the likelihood of rolling 12 on d12 is 8.33%, while for 2d6 it's 2.78% and 1.56% for 3d4.
If the original goal for the weight class dice assignment was to make the light vehicles least likely to have a high weight class and the heavy vehicles to be most likely to have it, then the 1d12/2d6/3d4 dice distribution does not support that goal.
The likelihood for weight class dice carries over to the Hazard Table as well - light vehicles are just as likely to roll any value between 1 and 12 while medium/heavy vehicles will tend to roll medium values between 5 and 9. Here it could potentially be the desired outcome, with heavier vehicles generally being more stable (?).
Apologies if this message comes of a bit dry, my intention was to be as informative with my findings as possible, I hope it helps :)