Road tar is highly adhesive and when left on a vehicle will eventually bond to the paint and cause etching or staining. A variety of  solvents may be use to dissolve the tar depending on the type of surface on which it is found, e.g. paint versus glass or aluminum. Once  tar is properly removed, the vehicle should be washed and waxed, paint sealed or both to protect the cleaned surface once again and leave a high gloss finish.