>> a Nissan Skyline, is much more likely to be modified with non-permitted modifications than the station wagon taking the kids to school. (a)

>> We know these cars are often modified, (b)

Note that those are not the same: (a) means "of all cars modified, more are Nissan Skylines than station-wagons"; (b) means "of all Nissan Skylines many are modified".

Police officers will want to know whether (b) is true, so that they can decide whether stopping Nissan Skylines disproportionally often is justified.

Unfortunately, (b) really does not follow from (a). For instance, (a) is true even if there are 0 modified station-wagons and a single modified Nissan Skyline. If only a small fraction of Nissan Sklyines is modified, cops are going to have to stop many, many Skylines before they hit one of the few modified ones.

Even worse, the only way to know how many Skylines are modified is to check each and every one of them (or stop "enough" of them and extrapolate).

On the other hand, cops will want to catch any illegally modified car. So they have to wonder how many other modified cars they let slip by if they're going after Nissan Skylines more than other cars.

If the cops wanted (or were able) to do proper data-driven policing or whatsitcalled, they should make a list of all the types of car they found modified in a particular time period, count how many times each type was modified, divide by the total and go after the ones with the highest ratio, and that only if the ratio is "high enough"- which again depends on exactly how many cars were modified in that time period.

But of course, all this is hard to do and costly, so cops will go by instinct and probably apply unwarranted prejudice- and get very sub-optimal results.