If you are an undocumented immigrant, you might not have a valid driver's license, and you might have to leave your car at the site or have it impounded. Some towns in the Peninsula don't allow traffic cops to ask for immigration papers or mix up their jobs with Federal immigration laws, - I don't know about Atherton. But if you're stopped and you don't have a license, the police can ask you to leave your car by the road, and you walk home. I teach ESL, and many of my students have had to walk away from their cars. If the police do impound the car, the owners often don't have money or are afraid to go get them, so they lose their car. This happens all the time, and often causes big logistical and financial problems for the families.