I lived in Paris in the late 80s, and the Bois was where I first discovered trans girls. It was the heyday of activity there, with several hundred girls working every night. Many were absolute stunners, and for 200 or 300 francs (roughly 20 or 30 quid) you could walk into the depths of the woods with one or drive off with her down a side alley of the bois. During the winter months, many would happily get into your car and drive around with you while gently warming their hands between your legs

At the end of an evening (around 3am), it was also possible to get a request to drive them home, which was often rewarded with an invitation to come up for a spot of tea

Parking the car, and walking along the paths of the bois was also great fun. Many of the girls would try to entice you to walk into the woods with them with a few well placed fondles between your legs. You did have to be on your guard, though, because many were fairly good at rubbing your manhood with one hand while the other probed your pockets for loose bills.
One of the great features of the bois was that you never had to worry about bait and switch. You could cruise until you saw a girl you truly fancied, and what you saw is what you got!
I personally never visited any of the vans, as those girls were always quite a bit less attractive than the ones flaunting it on the pavement.
Unfortunately, when Sarkozy became Minister of the Interior, there was a huge crackdown on both girls and punters. Although there has continued to be activity, it is a pale reflection of the total carnival party it used to be.
As for the question of safety, I personally never had any problems, but I suppose you need to have a bit of awareness and streets smarts about it.