Technically even with completely slick tires it would be possible to brake on ice, it would just require more time. As long as there's some kind of friction, there's no reason for an object to retain its speed, assuming it's not propelled in some way.
Now, I haven't personally tried it as I don't go on ramps, but if it's impossible to brake AND accelerate on the ramps, then they're defying the laws of physics ^^