I think many clients have their own version and opinions on what is safe, what is discrete.
I've had two potential clients say that they'd be recognised coming to an incall place, or booking a hotel room, so meeting in cars in a layby would be more discrete, and less likely to be spotted by the press:
In my opinion it's not safe, plus you have nothing to tie the person to that spot - with a normal outcall you have proof of residency, and often have spoken to them at a landline registered at, or belonging to the address (or hotel) to be visited. A man could send you to any location, and he be sat at the other end of the country, or even worse start playing cat and mouse, or have his mates drive up. Too many things to go wrong PLUS there is an offence of having sex in a public place AND I'd not like to take a chance that the police couldn't make a soliciting offence out of it - after all you are more or less operating on the street, when attending a car meet.