Question

__________ has the highest conviction rate of any vehicle-related offense.

Answer 1

The answer is "excessive speeding"

I would say that there are two causes for this: 
1) people speed more often than do other vehicle driving related crimes: that's because they explain to themselves that they are in hurry and they hope they won't cause any harm

2) Speeding is very often tested by the police, because the police wants to curb this deadly behaviour. 

Answer 2

Excessive speeding has the highest conviction rate of any vehicle-related offense.