From the Sierra Club:
Bicycles are not for everyone, and they're not for every trip. Cars do many things that bicycles cannot easily do: carry heavy loads uphill, protect riders from the elements, and cover long distances quickly. But a surprising number of car trips could easily be made by bike. Nearly half of all trips in the United States are three miles or less; more than a quarter are less than a mile.
Short car trips are, naturally, the easiest to replace with a bike trip (or even walking). Mile for mile, they are also the most polluting. Engines running cold produce four times the carbon monoxide and twice the volatile organic compounds of engines running hot. And smog-forming (and carcinogenic) VOCs continue to evaporate from an engine until it cools off, whether the engine's been running for five minutes or five hours.