Slowing down the game by means of CPU usage is highly discouraged, since it means your game is taking up 100% of the CPU, which slows down other processes, and is uncontrollable, and most importantly, acts differently on different processors.
A frame-rate limiter would be the best option, or if you had a Speed counter that you multiplied all velocities by before moving, then you could set the speed simply by changing the counter, which also allows for some things to not be affected by slowing down the game.
For example, a default speed of 100, then you multiply all velocities by Speed value(100) then divide by normal value(100), so if you set speed to 80, a car going at a speed of 5 would then go at a speed of 5*80/100, or 4.