My usual sympathies are quite libertarian.
But it should be a felony, punishable by a 20-year term and a $10 million fine to broadcast on the radio, either as part of an advertisement or normal station programming, the sounds of car horns, sirens, squealing brakes, or similar traffic sounds.
It was OK back when I drove cars with cheap and crappy sound systems.
But now I have a half-decent radio, and whenever some advertising asshats decide to call attention to their spots with a cacophony of sirens, my heart leaps for a few fractions of a second before I figure out that I'm not about to get run off the road by the entire Dover New Hampshire firetruck fleet.
Similarly when the local traffic report "cleverly" introduces itself with a cacophany of blaring horns, my immediate reflex is: Crap! What did I do? It's just realistic enough to dump some unwanted adrenaline into my bloodstream.
Next car I get will have a CD player. And I'll never listen to the radio on the road again.
Update (3/22/2005): It occurred to me that my legislation would need to make an exception for Expressway to Your Heart by the Soul Survivors. That's a cool Gamble/Huff song, and the traffic noises just make it better.