History of AASHTO Rail (Impact/collision) load?

I don't have any documentation on the origins of the 10 kip load, however, I have always used the 10 kips horizontal load for cantilever slab design, regardless of the truck loading (HS15, 20, or 25). I also check the vertical wheel load (for the design vehicle) but the moment arm is usually small and doesn't often control. I believe that the 10 kips rail load would be "code compliant" for all design vehicles listed in Section 3 of AASHTO, but you could always be conservative by multiplying the 10 kips by 1.25 (HS25/HS20).

If your interested, check out NCHRP Report 350 "Recommended Procedures for the Safety Performance Evaluation of Highway Features" which includes barrier rails.

Good Luck.

NCHRP 350 provides the crash test certification methodology. You can find increased barrier/railing loads in AASHTO LRFD section 13 and 13A. The loads in these section are based on instrumenting barrier/rail that was crashed under NCHRP 350. You'll find out that 10 k is too low. A lot of State DOTs are switching to the higher loads.

Good luck.