I was there yesterday and used single rider for Radiator Springs Racers. I went in the morning and walked right on when I went in the afternoon on Sunday it was 30 minutes.
The RSR Single Rider line is usually a good bet. It can get backed up, especially when very busy or if the ride has been going down, but if you don't mind being seperated it can be a very efficient way to ride the attraction. Other SR lines can be hit and miss.
There is, but sometimes they do close it off. My last trip I tried it in the afternoon, but they had it closed. But on the trip before, it was open in the morning when I went. May depend on crowds/time of day, but I would definitely try for it.