From the article:
Dogging refers to having or watching sex in a public place, usually outdoors. The term originates from men using imaginary dogs as an excuse for hanging around in the bushes. It attracts people from varied backgrounds, age groups, and professions, and it happens in parking lots, fields, and picnic areas all over the U.K.
Dogging Etiquette 101
The only thing I don't get about this is that cars are generally a LOT smaller in the UK. I realize I'm larger than average, but really - sex in a car is just not that much fun. Oh, and this is not illegal in the UK, whereas I believe it would be illegal in most states - in the UK someone has to make a complaint for an arrest to take place, while in the US the police can arrest you on their own judgement. *shrugs* Ah well, to each their own.