Signpost breakaway angle

[Tompar]

Hello,

I'm curious why for certain MassDOT signposts the breakaway flange is at an angle to the horizon.  Seems likes it increases the cost of manufacture versus a square cut so there must be a good reason.  I don't see this flange type on other DOT spec manuals, just MassDOT.  Anybody know the reason and history?  Thanks!

Look at drawing TR.5.2 page 186 of this PDF.
https://www.massdot.state.ma.us/Portals/8/docs/construction/1996Mconst.pdf

[PHLBOS]

MassDPW (predecessor to MassHighway/MassDOT) started using that breakaway post design with the angled flange in 1976. 

The likely reasoning behind it is due to when an errant vehicle strikes the post; the resulting rotating motion allows the sign to breakaway (the top flange sliding off the bottom flange) more easily & quickly than had the flanges been parallel to the ground surface or pavement.  The angled flanged are aligned with the respect to the direction of traffic.

Previous-generation D6/D8 signposts had no breakaway flanges.

[Alps]

I've never seen this before, and I've never thought to look! Now you've got me thinking about it. Not here in NJ though, we use U-channel posts.

[cl94]

I've never seen that, either. Breakaway posts in New York, whether U-channel, square, or I-beam, are horizontal/perpendicular.

[Tompar]

My thoughts were mostly in line with PHLBOS.  But being an engineer by training, I wanted to understand the lower level "physics" better.  This has piqued the interest of some of my colleagues in the office too and we're all scratching our heads.  Some other thoughts we had:

- The flange angle, and the resultant forces during a crash, increases the tensile loading of the bolts and the likelihood they will break faster.
- The flange angle encourages the sign top to roll and hit the ground AWAY from the car versus a horizontal flange that may break clean and the top of the sign rolls ON to the car.
- An angle cut on the post results in a larger weld bead length on the flange and thus a stronger joint.  (probably not the reason)
- Crud and stuff less likely to build up on a tilted surface versus flat (probably not the reason)

[roadman]

I've never seen that, either. Breakaway posts in New York, whether U-channel, square, or I-beam, are horizontal/perpendicular.

That's because New York uses a different system called BreakSafe.  In the BreakSafe system, the two sections of the posts are connected by frangible bolt assemblies instead of slip bases.

As for the MassHighway/MassDOT slip base system, PHLBOS has the reasoning absolutely right.  The slip base is angled to the most likely direction the sign post may be hit from.  95+% of the time, this correlates to direction of traffic.

[Dan]

https://www.massdot.state.ma.us/Portals/8/docs/construction/1996Mconst.pdf

this is going to make some fantastic bathroom reading.