Traffic Paint
Try "bug and tar renover" which is usually a proven blend of solvents that are safe for auto finishes.
If the paint is not heavy and only overspray, than the clay bar suggestion is the safest route.
For truly heavy build-up then straight solvents may be required.
Removal solvents depend on whether the paint is water or solvent based. For traditional solvent based paints try toluene or VM&P naphtha. Toluene is a better solvent for the coating but VM&P is safer for the vehicle paint.
For water-based coatings, try isopropanol or the above mentioned bug and tar remover.
Be very careful with lacquer thinner since this is normally a "stronger" blend of solvents such as aromatics, esters, and even ketones. This can soften and/or remove most automotive coatings if left on too long or rubbed too aggressively.