I have done car boots in the past and picked up work, as you said some can be scattered but is it a more productive method than say door knocking, leaflets or networking and the answer is no.
I have for a long time thought about supermarkets though, if you can set up a pitch outside the main doors with a couple of banners, some girls to hand out leaflets while you give quotes there and then.
The last weekend of the month is normally the busiest and most people who shop there are local to the area.