In 2009, the Manual for Assessing Safety Hardware (MASH) was adopted as an updated replacement of NCHRP Report 350. Under MASH, the impact severity of Test Level 4 (TL-4) (Test 412) was increased by 56%. Test vehicle mass was increased from 17,640 to 22,050 lb. Impact speed was increased from 50 to 56 mph. Although this is a significant increase in the amount of lateral energy imparted to the barrier, the 54-kip design impact load currently suggested by the AASHTO load and resistance factor design (LRFD) bridge specifications has not been updated to meet MASH TL-4 requirements. Similarly, the AASHTO LRFD specifications require all TL-4 railings to have a minimum height of 32 in. However, because of the increase in test vehicle mass and impact velocity under MASH, the minimum height for TL-4 railings is expected to be greater than 32 in. This height was verified by a recent unsuccessful MASH Test 412 of a New Jersey profile rigid concrete barrier 32 in. tall. The research presented in this paper used finite element analysis and subsequent crash testing to determine the minimum rail height for MASH TL-4 impact conditions. The researchers also determined the appropriate lateral impact design load for use with AASHTO LRFD bridge specifications for the design of TL-4 bridge rails.