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I was reviewing the noted report today before opening your recent email. I wish that Roger would have shown this report and test video at the AASHTO TF13 meeting last month. He did show TTI’s 2270P crash test on the IA transition without curb, which rolled over under MASH TL-3. I do not have this report though. However, this test report demonstrates that vehicle instability may not be a function of the ¼-post spacing configuration without a curb but rather that instability is more correlated to magnitude of wheel interaction with the barrier end. Since the new TxDOT transition uses ¼-post spacing without a lower curb, then I would believe that more significant tapered end section would allow for the IA transition to work without curb. This opinion is also supported by the fact that several other crashworthy thrie beam transitions attached to concrete ends that utilize ½-post spacing without a lower curb have contained pickup truck impacts under NCHRP 350, although they include tapered bridge end with a larger lateral offset at the upstream end.

So what would you recommend for a lateral offset to use at the bridge end?  And what taper rate to bring it back to full width?

A 3:1 taper would seem reasonable and with upstream end at least 6 in. away from front face of rail.

The new TTI system utilized a 3:1 flare with a 6 in. lateral offset. A prior NDOR system utilized a 3.2:1 flare with an 8 in. lateral offset. CALTRANS had a system with a 1:1 flare and a 6 in. lateral offset. All of these tests were successful with pickup truck testing under either 350 or MASH and no curb leading up to concrete parapet.