Americans tend to accept both empty parked cars and occupied moving cars nearly everywhere, but disapprove of occupied parked cars, especially when the occupants are asleep. Therefore if you plan to sleep in your car, and do not need the facilities provided by a campground, you need to think about where to sleep without being ticketed or harassed.
The best place to sleep is obviously one where no one will come by and see you. For example, many of Colorado’s dirt roads are rarely patrolled or even used, especially during the week, so you can simply pull off the road and nod off. If such a place is not available, the next-best bet is to sleep where no one will notice you. If yours is just one among many cars, you avoid window condensation, and you don’t stay in the same place too long, no one will notice your transgression. The overnight shuttle parking lot in Aspen is one such place. Finally, you can choose a place where no one cares if you are sleeping in your car. Most Eastern Sierra trailheads qualify, even the crowded Whitney Portal.
By far the easiest place to rest in peace is National Forest land. Thanks to shrinking budgets and massive scale, there are few rangers patrolling them for miscreants. Also, the “multiple abuse” land management policy means cars are allowed to drive and park in more (and more out-of-the-way) places.
National parks present more of a challenge. They are better-funded and smaller, and cars are restricted to small, well-developed areas, so it’s hard to find a place to park out of sight. As always, arriving late and starting early helps. Parking in the long-term backpacker lot is best, but you can often poach a campsite if you get an early start. A late (after 9PM or so) arrival also avoids park entry fees, even at major parks like the Grand Canyon.
When sleeping on the highway, it’s best to do what long-haul truckers do: they sleep on the road more than you do, so they know what they’re doing. On rural interstates, you can just pull off at a local road exit and park on the side of the road. Many rest stops are also good places to sleep — look for rows of parked trucks at night. On smaller highways, pulling well off the highway on a dirt Forest Service road usually works. Finally, you can always simply park on the side of the highway. While your rest may be disturbed, no policeman will ticket you if you say you were worried about falling asleep at the wheel.
It is best not to sleep in town, but sometimes it can’t be avoided. If you are forced to do this, a motel parking lot is often a safe place to spend a single night. Just don’t park in front of someone’s room or near the front desk. Sleeping in a residential area is usually a bad idea, but if you must, choose a safe area where street parking is normal and few people are out late at night. Wealthy or upper-middle-class suburbs usually qualify, since everyone will be inside watching TV or sleeping, and the houses are far enough apart that an unfamiliar car will probably go unnoticed for a single night. Finally, remember that you are not breaking any laws by parking on a public street. The worst the police can do is ask you to move on.
Leave no trace!
Remember to follow the same practices and courtesies you would in the backcountry. Park a discrete distance from other people, pack out your trash, and don’t leave any human waste (or, if you are far from civilization, dig a cathole like you would in the wilderness). If you leave no trace, most people won’t mind that you stayed the night.