I doubt that there are less drunk bus or truck drivers.

This seems strange to me. Why do you doubt this? Even if bus / truck drivers drive drunk at the same rates as car drivers, there would be less drunk bus / truck drivers due to the numbers.

The absolute number is whats important because each drunk driver you add is extremely dangerous to society.

Also if society moved away from car driving, the loss in car vehicle miles traveled would not be compensated for by equal increases in truck / bus vehicle miles traveled because trucks and busses aggregate many trips together.