Here in the UK there is no law to stop you sleeping in a motor vehicle, so I do not know where that came from. Lay-bys are there for having a rest/sleep, that is why you see the lorries parked up overnight. No stopping on motorways for a kip, though; and don't use the parks at service areas, for if you sleep longer then the permitted staying time, then they try to fine you with a supposed 'parking fine ticket.'