In many countries, motorway parking places sometimes double as (gay) cruising spots. In short, these are places where people (often, but not always, closeted male homosexuals) go to have sex with one another. Most often, these places are well-known by the locals, of course by the people who use them, but not always by the inconspicuous hitchhiker.
Needless to say, it's best to avoid these places. Besides that you might run the risk that people might solicit you for sexual acts (although rape is extremely rare; - at least in the Netherlands, authorities tend to look the other way and allow these places to exist as long as the users cause no trouble), it will be very hard to get a ride from there. This is because if a parking place becomes too infamous for these activities, people who are not interested in these activities tend to avoid them. There is also the other risk of being targeted by people who are anti-gay.
Motorway parking places used for these activities tend to be secluded, usually with a large amount of bushes surrounding them or completely in (or with access to) the forest. They may also be spotted by a large number of cars being parked there, but very few people on the actual parking place.