Try Dr. Bronner's Peppermint Castille Soap. It can be found at health food stores and in addition to being great for use as a body (human) soap, it works a treat removing bugs, tar, and that wierd sticky road funk off windshields.
The label has this strange religious diatribe on it, which makes for great bathroom reading..
Great product, and man, does it dissolve even the toughest bug layer.
Chip