For a typical parking spot in Toronto the rain would land on the pavement, picking up contaminants from vehicles and then wash down the nearest storm drain. This gives the water an expedited route, straight into Lake Ontario. This water is untreated and is a great problem for our water ways in terms of pollutants and also because the rainwater heats up as it passes over the pavement it greatly increases the temperature of streams. This is bad for fish.
On a recent trip to BC we saw a neat way to manage rainwater on a parking lot.
The parking spots slant down to a vegetation area at the front of each vehicle. Try not to notice the shadow!
The vegetation is a mix of special plants and bacteria to break down pollutants and serve as a mini treatment facility for rainwater that otherwise would have gone down a storm drain.