By driving up close on civilian cars you build up the boost meter. Instead of trying to approach them without direct contact (which can be hard to perform) simply crash into a car that comes your way (it must not go in the same direction as you do). By doing so you will only lose about 40 km/h of speed but gain an additional full turbo boost.
Gamelati 1000 SS Motorcycle
Complete all eight road challenges.
Gameloft F1
Complete all eight road challenges.
Lamborghini Gallardo police car
Complete all eight road challenges.
Quick refill of the boost meter
On the roadside are various objects like phone booths, hot dog booths, plants, street signs, boxes, etc you can drive through. Doing this gives your turbo meter a nice boost.