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We have been having a discussion on the orientation of triangular slip bases with relation to traffic.  MoDOT’s standards have always shown the flat side of the triangle should be facing traffic (see below), we assume this was done so there are two bolts on the same plane as the sign on the sign side of the post to help resist the wind load from passing trucks and snow loads from plows, not having to do with breakaway performance since this is supposed to be breakaway in all directions. 

We have some vendors who make similar breakaway slip bases and their installation instructions indicate the point of the breakaway should face traffic.  When questioned about their orientation they indicated that direction came from the way they tested the device.

From your experience, is there a preferred orientation for triangular breakaway devices which may affect the breakaway performance or strength of the installation?  Having two orientation based on what device is being installed is causing confusion with our crews and contractors so we were hoping to establish a uniform installation regardless of the product.

Thank you for any insight you might be able to provide.

Our team performed a quick review of breakaway poles and signs and found guidance in FHWA Eligibility Letter No. LS-16. It appears that there were two preferred orientations and one ”worst breakaway performance – not recommended” orientation. Now, the guidance pertained to specific devices. If actual crash testing and guidance was provided to another device, I would utilize that guidance that was demonstrated. Please note that the LS-16 guidance is very old and may not pertain to MASH tested systems. Further study may be required to adequately determine the MASH recommended orientations.

From a design standpoint, it is important that the design of the sign, pole, slip base mechanism be configured to accommodate the wind loads regardless of the orientation. Further, any full-scale crash testing should be performed on the critical orientation of planned orientations in order for all other planned orientations to be acceptable.

Please let us know if you have any other questions or comments. Thanks!