First of all, they’d probably all fit in one lane- but they would use them all anyway I reckon. If it was a one-day phenomenon then there would be a ton of bicycle accidents, since most of those riders would be inexperienced or inept. Car and truck traffic would crawl at best, since inexperienced cyclists would not know how to interact with the vehicles and most drivers are clueless about interacting with bicycles (or any other vehicles). If it was a regular thing, then the city in question would get to look a lot more like many cities in Asia and Europe where many folks use bicycles for transport, except that it would have wider streets. However, I think even at $10 a gallon most American city-dwellers will drive as much as they can afford to… and of course suburban burghers have little choice.