Shea car footprint bug

Ok this is a quick fix to a bug I noticed, which is that because of the way I implemented motion controllers it reset every vehicle's dimension to the default value after the first iteration (causing all vehicles to be of the same dimensions).

This fixes that so they keep the dimension they have at the first iteration throughout the scenario. This may be important if we want to experiment with injection agents of different sizes (which might be a good idea to cut down on injection locations - just inject like a truck over the entire lane).