The rails are not designed for head-on collisions; they’re designed for glancing sideways swipes. We actaully had a guy here in Austin hit a bridge rail head on at high speed about four years ago and he wnet right through it. The case was kinda weird tho’, it was an overpass that ended in a T and I think they determined the guy was drunk. I wish the article was still online, there was a great picture of a bunch of TxDOT engineers standing aroudn looking at the hole going “yep, it’s busted”.
The sloped concrete rail (either single slope or double slope) is actually designed to guide the car or truck back down to the road. Metal beam guard fence (the metal rails usually on large wooden posts) provides a transition from bridge railing to open road or is used in places where you just need a little protection (i.e. the drop-off isn’t horrible). The type of rail you use depends on the road’s design speed, the expected traffic, and possibly some aesthetic requirements.
All the bridge rail used in Texas is crash tested by the Texas Transportation Institute located out in College Station at Texas A&M. Here’s list from the TxDOT Bridge Railing Manual on the types currently used in Texas. Notice each type has a recommended design speed; that’s the speed at which the rail has been tested. I’ve seen footage of the crash tests and it’s pretty fun. At this very cool link they have some little quicktime movies of various crash tests. Notice they don’t test the rails for head on collisions, only for angled ones. If you want to see a slightly more detailed reoprt on crash testing, here’s one. (Warning: large PDF)
One of the interesting fairly recent development in rail testing concerns the type of vehicles used. Formerly passenger cars were mainly used for tests. But with the growing popularity of pick-ups and SUVs, the researchers realized that soem of the existing rails weren’t adequate. Trucks have a higher center of gravity and a rail that works for a car might not for a truck. So the standards have been revised.
OK, I realize I didn’t directly answer your question about how fast you have to be going for a head-on collision. Truthfully, I don’t know off-hand since it’s not usually a consideration (though I could calculate it if I really wanted to). The guy that went through the rail here a few years back wasn’t going all that fast, maybe 35mph but he didint’ even try to brake before he hit the rail.