In our Design Manual, we currently state the attached regarding crash cushions and curbs.
Should we remove the portion regarding curb conflicts and advise crash cushions not be placed near curbs, or does this guidance still apply?
There is very little guidance regarding crash cushions and curbs.
As far as I know, the largest study in this area was undertaken by CALTRANS years ago. The study looked at sand barrel arrays and curbs. CALTRANS performed several curbed gore area vehicle jump tests and a full-scale crash test with a 2,790-lb passenger car impacting a sand barrel crash cushion on top of a 6-in. tall curbed gore area at a speed of 41 mph. The sand barrel crash cushion was 5 ft behind the perpendicular curb, and the curb did not adversely affect the crash cushion performance. It was recommended to place a sand barrel crash cushion 0 to 5 ft or over 50 ft behind a perpendicular 6-in. tall curb and 0 to 3 ft behind a parallel curb (the side of the gore area) if a curb was necessary. Aside from that research, no full-scale crash testing of curbs and crash cushions exists to the my knowledge.
The RDG Section 8.4.6 denotes that the presence of curbs can later the vehicle trajectory and thus affect the interaction of a vehicle with a crash cushion after contacting a curb. It recommends that curbs not be used for new construction where crash cushions should are to be installed. Existing curb and crash cushion installations should be reviewed to determine if they have posed an issue. It also notes that a curb no higher than 4” may be considered acceptable on existing construction or left in place unless past performance has indicated and issue with the installation.
It appears that your guidance is a combination of these recommendations.
I believe that there is an upcoming NCHRP problem statement related to this, but I don’t believe it has started yet. We are also studying end terminals with curbs in the Midwest Pooled Fund. That may shed light on this subject.