One thing that has always bothered me was that the streets in Liberty City weren't as packed with parked cars as they would be in real life. I did a test run with some of the stream wpl files inside the game map's img files (in this specific instance,
brook_n_stream2.wpl inside of
brook_n.img), and this is the result I came up with after a couple of additions:

I want to emphasize that those cars were not manually spawned! I simply drove from the safehouse in Hove Beach to downtown Broker and the cars were all there.

Also, I have a modified popcycle.dat file with higher values in the "parked cars" section in order to ensure that all parking spots get filled.
This has the potential to become a great map modification to add realism. Don't know how the added parked cars would impact game performance, so that would need to be explored. Also, it would take quite a while to create, since there are tons of streets in the city. Might finish it just in time for the release of GTA V!