SKIDTHRESHOLD should be a value between 0 and 1, with values closer to 1 requiring less intense skidding to produce marks. SKIDINTENSITY can theoretically be anything, but I recommend values between 0 and 100, 100 being the most intense. As it is right now, the opacity of the skid marks is a function of tire grip and SKIDINTENSITY, so you get a gradual fade in as the skid intensifies, but you could simply set the opacity to SKIDINTENSITY directly for a more instant effect.