Haha... got confused for a while... the signs do not bode well for me on the GMAT...
Anyway, to simplify, we'll assume 100 people drive on this road.
20 percent of violators do not receive tickets, therefore 80% of the violators do.
We know those who receive tickets consist of 10% of all drivers on this road, so 10 people. In other words, these 10 people make up the aforementioned 80%.
100% of violators would be 10/0.8 = 12.5, so 12 people & half a bloody corpse, or 12.5%.