extremely safe. much much safer than driving in any case.
The closest ive got to another glider is when i went flying on a ridge in Scotland. The wind blows against the hill and is deflected upwards, so if you stay in the rising air you can stay up for as long as you like. There were about.. 15 or 20 gliders (including myself) on the ridge (which is about a mile long) going up and down it, and you have to avoid each other. Its pretty simple really, you just have to keep a good lookout of what's around you... and it's amazing fun flying about 50ft from the hill at 60mph with everyone else there as well. There are accidents, but they are easy to avoid. Compared to the number of car crashes, its nothing.
If the worst comes to the worst though, and you do hit someone, you can bail out and parachute down.