I think that whatever the barrier, it is the deflection angle that needs to be considered. Hitting parallel is going to be bad due to rapid deceleration, so if it is possible, it should be angled to deflect from parallel (like a castle wall is angled to deflect a cannon ball), but the car should not be 'bounced' back on t the track to collect other cars.
Although we would know the angle the car is passing down that section of road, it would not be possible to place barriers at angles to it unless the run off becomes unseasonably wide at the far end. The alternative would seem to be a barrier that collapses in a controlled manner.
I think I recall seeing barrier filled with water, where the impact is transferred to squishing water through small openings. This presumably would automatically create the angle by deflating?
This seems like the best option to me, but there are probably many problems with it that do not come readily to mind ( such as freezing etc)
Has anyone here had any real experience with these water filled barriers?
We are standing on the shoulders of Giants. So don't kick.