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The New Jersey Department of Transportation (NJDOT) is currently developing standard details for a concrete balustrade railing. We are considering a railing design (see Figure 1 below) that has been successfully crash-tested to the MASH TL-4 level (eligibility letter B285 is attached for your reference).

Figure 1 – Typical Section of Balustrade

Our main question concerns the transition of the vertical railing face near the trailing end pylon.  The vertical rail elements of our design are set back 6 inches from the front face of the top and bottom curbs. Our current understanding of the roadside safety community (MwRFS) recommendations suggests that a 10:1 taper should be used for the horizontal transition of railing faces to prevent vehicle snagging.  If we strictly adhere to the 10:1 requirement, we would need to introduce a 5-foot-long transition section before the trailing end pylon, as illustrated in Figure 2 & 3.

Figure 2 – Balustrade to End Pylon Transition

Figure 3 – Railing Setback Distance

We believe that because the vertical rails are recessed from the main curb face of the barrier, a 10:1 horizontal transition may not be necessary to prevent snagging.

We would appreciate your expert guidance on this design detail to ensure the standard is compliant and safe. Please let us know your recommendation at your earliest convenience.

Thank you for your time and assistance.

I think we can provide some insight here. In terms of the tapered end on the barrier adjacent to the trailing end pylon, we believe that tapering is necessary in this case as the end of the pylon would provide a vertical edge for vehicle snag as an impacting vehicle transitioned off the bridge rail. This vertical edge would extend farther forward that the aesthetic openings in the railing and would have significant snag concern.

That said, we believe that we can shorten the tapered section significantly. We recently did research for the Minnesota DOT on a combination bridge rail. In that research, we looked specifically at this type of end taper. That railing had an 18” tall lower parapet (3” shorter than your system) and an upper steel tube rail. The snag concerns on the trailing end pylon were very similar. In that study, we applied an end taper moved back 9.5” over 22.5” longitudinally. This resulted in a taper of approximately 2.37:1. MASH TL-3 1100C and 2270P tests were conducted directly on the tapered section and the results were acceptable.

Based on this testing, we believe you could use a similar taper in your bridge rail. In order to apply this shorter taper, one would need to maintain equal or less longitudinal spacing between the final balustrade post and the start of the tapered end section to ensure that the potential for vehicle overlap on the tapered end is similar to the overlap at each balustrade post in the middle of the system.

So while we can’t recommend removal of the tapered section, we can recommend shortening it significantly.

https://mwrsf.unl.edu/researchhub/files/Report431/TRP-03-403-21.pdf

Thanks!