I am sure that the lines on the sreet are equally as slick as the lines on a track. I know that the lines on the roads around here (Montreal) are slippery, as I often get the back wheel spinning when I change lanesd and cross a line. You are just not going as fast, or on the brakes or throttle as hard, when you are on the street. When you are on a track, you are using a lot more of your tires' traction, so when you cross a line, you are WAY beyond the traction the painted line can provide. Most of the time when you cross a line of the street, you are probably not asking for more traction than the line can provide.