Me I use the SHEEWEE, It's bloody brilliant, the last time I used it was into a bush on the hard shoulder somewhere along the North Circular, it was so easy to use and now I keep one in my bag at all times. I even bought one for all my friends one Christmas..
They really do work, the best story I have for you is the one where I was at Longleat Safari Park, with my family and we had gone half way around, and I wanted to pee urgently, obviously we couldnt stop or open windows as the wildlife would have had little sympathy for a middle aged woman with a weak bladder, so I had to scramble over to the back of my 4-Wheel drive over three teenagers and into the boot , and I managed to pee into an empty evian bottle. There is a video of it, but I daren't show you, as it is far to embarassing.
There are others on the market and here they are, but I would invest in the Sheewee, get one now