In all, there are nine locations where you can drop off/pick up people. These are:
Metroville Cutters' Lodge
Jimmy Jane's Coffee Shop
Mayor Cole's mansion
When you have a passenger that wants to go Kamp Krusty or the Interesting Home and you are near KBBL/Quimby's mansion: Cut through Quimby's mansion and drive through the water in the back. Drive to Kamp Krusty/Interesting Home this way, it saves a lot of time compared to driving on the main dam.
When you have a passenger that wants to go to Rancho Relaxo: Go on the dam then immediately drive off of it and go into the water. Continue driving in the water until you see a path to your left. Take the path up to the main road and Rancho Relaxo will be across the street.
-Note: When you have finished dropping off the passenger at Rancho Relaxo, immediately go back to the main road near the path. There would be passengers that would either want to go the Mayor Quimby's mansion or the TNT storage. When you pick them up, go and drive down into the water. Continue to follow the river until you see the Springfield Dam. Speed up the ledge of the Dam so you can jump across it. Once you jump across it, you are on the other side of the dam that is behind Quimby's mansion. From there you can take the passenger to their destination [follow the hand]. This saves a lot of time.