1. Sign, Signal, and Luminaire Supports, Utility Poles, Trees, and Similar Roadside Features
notes
RDG Chapter 4

2. Roadside Appurtenances
That sure is a mouthful, huh? You can think of this Lessons 4, 5 and 6 as addressing most “vertical” hazards. We are talking now about things like sign supports, light supports, utility poles, and trees.
Sign and Luminaire Supports
Traffic Signal Supports
Mailbox Supports
Utility Poles
Trees

3. crash This happened in New York in the early 60’s
One of the reasons we take all of these measures is this -- to reduce fatalities and accident severity. It isn’t uncommon today to find a non-breakaway device on the roadside.
The original requirements for breakaway devices were in the AASHTO standard specifications…

4. Stub Height Criteria Stub 4” max. 5’ min.
Another requirement is that after a vehicle impacts a breakaway support, the stub - the part that is left in the ground after the crash - can be no higher than 4”, measured at its highest point in relation to a 5-foot long chord. This is to prevent vehicles from snagging an axle or rupturing gas tanks as they pass over the stub.
The slope hinge point is usually the highest potential snag area. This is where the stub height will be the most critical, especially if one side of the car is over the slope and the other side still up near the shoulder of the road.
Stub
4” max.
5’ min.

5. Sign Supports

6. Types of Sign Supports Overhead Signs Large Roadside Signs
Small Roadside Signs
There are basically 3 types of sign supports. If a sign is massive, it should ideally be placed on a overpass to eliminate the need for separate supports or for shielding.
Most other supports can be made to yield (breakaway) when hit.

7. Portland/Oregon street sign
Here is an example of eliminating the hazard -- we’ve installed the signs on a nearby and convenient overpass.
The signs are easily visible, and we haven’t created anything hazardous.

8. Tysons/Lessburg street sign
This is a mountable curb in an area of relatively high speeds and potential traffic conflicts.
We could have used a breakaway ground mounted sign here instead of this large cantilevered support. In any event, this sign should be shielded. The curb has no redirectional capability.
Recommendation for ground mounted is not always pratical. Other options are barrier, crash cushion, or multiple breakaway supports.

9. Ground-Mounted Breakaway Signs
Wind
Ground mounted breakaway signs are designed to resist wind and ice loads, and yet allow a vehicle to strike the support and have the support break away. The sign is strong in moment (bending at ground line) to resist the wind load, but weak in shear to allow the post to separate from the base when hit near ground level.
The dead load and ice load is straight down. The base needs to be designed to be weak in shear so that it ‘breaks away”
Vehicle Impact
Weak in Shear
Strong in Moment

10. Car position 1

11. Car position 2

12. Car position 3

13. Car diagram
Note that an upper hinge is needed for this type of design and that it must be at least 7’ high -- this is so the sign clears the windshield and roof of the vehicle.
The support wouldn’t work right if it was lower. 7’

14. Uni-directional slip base
This is a uni-directional slip base -- it will release as designed when hit head-on. The bolts should not be tightened beyond a specified torque; otherwise, the slip base won’t work smoothly. There is also a thin-gauge “keeper plate” which prevents the bolts from falling out of their slots from vibrations caused by wind or traffic. When hit, the bolts tear through the keeper plate fairly easily.

15. Frangible coupling
This is one type of a frangible coupling. Depending on the upper hinge design, it can be considered a multidirectional support. Although this version is designed to work best when struck head-on, the latest design is truly multi-directional. These couplers use necked-down bolts which allow a stress concentration and fracture at that point -- but you don’t have to worry about bolt torque or from which direction the vehicle is coming.
Through a combination of shear and moment the bolt breaks away. The nob on top of the bolt is designed to take the wind load.

16. Old Mill Bottom Road Sign
Here’s an example of a good location for a sign support -- it’s up on a cut slope and probably won’t be hit unless the vehicle happens to be flying.
The AASHTO specs state that you shouldn’t put the signs below the hinge point for two reasons....

17. Lower signs
...it stiffens the leg, and the lower signs will most likely be at windshield level. So the lower signs will go through the windshield instead of over it.
If you look in the background -- there is a breakaway luminaire support on a non -breakaway concrete pedestal -- that’s a little higher than 4”.
This will probably work fine if the vehicle runs off the road at the luminaire. However, that support is in the ditch line. So, if a vehicle ran off the road at an earlier point -- it would get funneled in the ditch and be stopped suddenly by the concrete pedestal.

18. Small Sign Supports Base Bending Slip Base Fracture
Small sign supports are those used for your garden variety regulatory and warning signs like stop signs, yield signs, speed limit signs, curve warning signs, and route markers.
Base bending or yielding supports are generally U-channel steel posts.
Slip base designs, which are similar to large signs but without an upper hinge, can be either uni-directional or multi-directional.
A fracture type support would include wood posts, or steel posts connected at ground level to a separate anchor.
All of these function by bending flat or snapping off at the ground when struck.
Base Bending
Slip Base
Fracture

19. Breakaway Hinge Point on Slopes
Another thing to remember -- when a vehicle leaves the road at speed -- it will be partially airborne. You want to avoid putting a breakaway sign at a location where it gets hit too high -- because it may bend instead of breaking loose from its base.
So, on a 1:4 or flatter slope -- you need to have at least 20’ beyond the hinge point of the slope, so that the vehicle is back on it’s wheels. A 1:6 slope would be much better.
Safer Roads
Roadway 7’
20’ 1 4

20. 2 concerns
There are two concerns here - one obvious, one a bit more subtle. Can you spot them?
The concrete footings are higher than 4”. Some vehicles could snag on the concrete and not even get to the slip base. This is one of the most common installation problems with breakaway supports!
Note also, that the upper hinge design makes this a unidirectional support -- it will only work well from one direction. Because this sign is in a median, it could be hit from the back and the support won’t work right. The larger the sign support, the greater the concern for “wrong-way” hits.
See Fig 4.7 for bi-directional support.

21. Luminaire Supports

22. Types of Breakaway Luminaire Supports
Like most vertical supports, our design option will generally include making it breakaway.
For luminaire supports, most states use transformer bases that are cast aluminum and shatter when they are hit.
Some states use slip bases or frangible couplings similar to designs used for large sign supports. And…
Some cities and counties use fiberglass poles because they come in pretty colors.
Transformer Bases
Slip Bases
Frangible Couplings
Fiberglass Poles

23. 4” Stub Requirement
Luminaire supports basically work on the same concept as sign supports -- they also have the 4” stub height requirement. (See the sign support lesson for more details.)
This is an example of the cast aluminum transformer base.
Usually, these supports are placed on a concrete base -- you need to make sure that the base doesn’t exceed the 4” stub height requirement.