This was my experience as well!
There are only 3 single rider lines anyway and imho its often not worth it on RnRc, can be a time saver at TT and usually much quicker at Everest. At test track if you want to design a car just go through the gift stop toward the exit and there is a design studio there. Design a vehicle, tap your vehicle onto your MB and then you can use that vehicle when you ride single