The ability to specify various speed blocks, creating an association between the Material and the boost that should be gained from driving over it.
The speed will be stored as a modifier i.e. "0.1" would be 10% of the original speed, "1" would be the original speed, "10" would be 10x their speed etc.

When driving over a material such as GRASS, this can have a negative effect on the speed (to stay on the intended road for example).

After experimenting, I've decided that the speed modifier will belong to the actual Car object, by increasing it's currentSpeed, it's then the Car's responsibility to gradually bring the speed back down to the limit.