Where does "sap" come from?
Firstly, it's well worth pointing out that tree sap is not actually from the trees dropping sap, it's from a number of different varieties of insects that are feasting on the tree. They call the dropping residue the nice name of honeydew, which is just another name for bug poo! These insects really become active and swarm in the warmer months, much to our annoyance on that lovely warm spring day!
Just don't park under trees
Prevention is always best, so the ways to stop this happening is to not park under trees (obviously), if the park is at your home, look at investing in a carport, or getting rid of the offending tree (not always practical). You can look into getting rid of the insects, with a pesticide (usually not good for the trees) or with other bugs that eat the tree eating bugs, with ladybird bugs being a popular one with many female gardeners.