But come to think of it, that'd be a little weird: you'd be able to drive your car into the city on one day, but wouldn't be able to drive it out the next.
They did this in San Jose, Costa Rica (and maybe they still do, I don't know). Cars were each restricted one weekday. Plates ending in 1 or 2 on Monday, 3 or 4 on Tuesday, etc. It wasn't 24 hours, it was from ~6am to 8pm.
It was somewhat successful, though not surprisingly considerably less than a 20% reduction. Taxis were not restricted, and of course wealthier families tended to have more than one car.