The reccomended space/speed is 10', for every 10 mph of speed. Although... I have found that leaving that much space is an open invitation to every Indy driver wannabe to try and squeeze in, in front of me, creating a dangerous situation. To avoid this, I try not to tempt people, by leaving, about a car length of distance to the car in front of me. I wish I could leave more room, but there are to many wankers out there.