I'll bet today's modern car horns are more efficient than "back in the day".
A horn shouldn't really use much juice. If you're not trying to pump out low frequencies, very high decible levels can be made with only a few watts.
BTW, here's an x-tra loud Harbor Freight horn:
Harbor Freight Tools - Quality Tools at the Lowest Prices
My brother is a freight train mechanic for Canadian National (used to be Illinois Central) railroad. I asked if he could get me a scrap piece of exhaust gasket material for my bike engine. He gave me a piece about 4" x 8", enough for half a dozen gaskets (train parts are big), and it was the baddest gasket material I ever saw. It consisted of layers of teflon bonded to thin stainless steel. No fun to cut. I had to use my Dremel tool with little cutting disks. But I can take my muffler on and off a thousand times and that gasket won't leak a bit.
Oh, I got off topic a bit. I thought about trains because their horns are so incredibly loud. My brother told me that they don't joke around with blowing those horns indoors because they can ruin someone's hearing instantly.
On a lighter note, could you put a semi-truck air horn on a bike and power it with a CO2 cartridge?