A decent method is to:

1) limit the steering as you gain speed

2) apply a downward force relative to the vehicles orientation upon turning.

A roll bar may work, but it depends on the implementation. If it's invisible and sticks out very far it'll catch things and toss you around. In this situation it's best to take a cylinder entity, apply it to the vehicle via a looser hinge joint, and have it set to the same collision group of the vehicle(passable unto each other). So long as the hing joint is loose enough you shouldn't catch very often and will have a nice "rollbar" radius...