Are there any specific steps for "Traffic Enforcement" achievement, or is it bugged? Tried it twice, and the car always drives away when I approached.

I shot some dudes and put them in the van then somehow became a cop somehow and the guy drove off then next scene the truck I tried pulling over looks abandoned on the side of the road with cones around it and no person in it or no where to go. Is that the end or a bug?...