Utility pole location

[ACtrafficengr]

I hope you folks will forgive a question from a traffic engineer, but why is it that utility poles are often placed on the outside of highway curves? Sometimes, it seems the road is intentionally crossed to do so.

This is a serious question. Every year, over 1000 people in the U.S. alone are killed in collisions with utility poles. Placing poles in a place they are more likely to get hit does not make sense to me.

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"Come to think of it, there are already a million monkeys typing on a million typewriters, and the Usenet is NOTHING like Shakespeare.

Blair Houghton

 

[davidbeach]

Curved pole lines need to be guyed. If the pole line was at the inside of the curve, the guys would be toward the road, or the poles would have to be braced.

 

[tinfoil]

If the overhead wires on a pole 'turn a corner' of more than 3 to 4°, then the pole has to be braced by a guy and anchor, with the 'lead' (or distance from the base of the pole to the anchor point at grade) being typically 1/2 of the attachment height on the pole. {These wires are often each tensioned to 3000lbf to achieve minimum sag requirements}.

So for a typical utility pole, this guy and anchor will be 15 feet (5m) or more from the base of the pole. If the pole were on the 'inside' of the turn, the anchor would be onto the road allowance (maybe onto the paved portion). Obviously, this is unacceptable.

Therefore the utility then has to run an overhead crossing guy to another pole on the far side of the road which will have the 'down' guy to satisfy the first pole's guy requirements.

Therefore, we still have a pole on the 'outside' of the bend in the road, PLUS a pole on the inside of the bend: More costs, and TWO obstructions on the roadside.

Locating the pole on the outside of the bend is actually LESS obtrusive to traffic.

 

[ACtrafficengr]

Then why are breakaway pole bases so rare? They give way to lateral impact loiads at the base, but withstand the moments developed by wind loads - sometimes better than a straight wood pole!

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"Come to think of it, there are already a million monkeys typing on a million typewriters, and the Usenet is NOTHING like Shakespeare.

Blair Houghton

 

[jghrist]

Then why are breakaway pole bases so rare?

Cost. Breakaway pole bases would only be applicable to steel poles with concrete foundations. These are much more expensive (even without breakaway bases) than wood poles or direct embedded steel poles.

Guyed poles on curves do not have to withstand moments. The loads (mostly from wire tension) are held by the guys; pole loads are vertical column loads.

 

[ACtrafficengr]

Breakaway bases have been developed for wood poles. They have been shown to reduce injury to vehicle occupants and damage to wires from crashes. See

http://safety.fhwa.dot.gov/roadway_dept/road_hardware/listing.cfm

Also, I was referring to moments from wind and ice loads to unguyed poles. A test site in Massachussets survived a nor'easter that took down the conventional poles on either side.

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"Come to think of it, there are already a million monkeys typing on a million typewriters, and the Usenet is NOTHING like Shakespeare.

Blair Houghton

 

[stevenal]

I've seen pictures of a wood breakaway pole. The application was a tangent structure where the adjacent poles would continue to support the conductors which in turn supported the top part of the broken pole. I can't see how you would apply this to a corner pole. All would come down, live conductors and all. Now the risk is not just to the drunk who failed to make the corner, but to other motorists, pedestrians, rescue workers, etc.

 

[tinfoil]

Mind you, the feeder protection is SUPPOSED to trip the feeder if the wire is in contact with the ground....

... if the protection is working correctly
... if the impedance of the wire-earth contact is low enough
... if the wire is actually on the ground and not just lowered
... if the distribution system is not delta-connected

Bottom line: utilities design their systems to keep the wires way up high where the public can't touch them. Here in Ontario these poles may be supporting voltages of 44,000 Volts. If I have to choose between protecting a careless driver from self-injury, and protecting everybody else, guess which way I'll choose?

 

[stevenal]

Lots of ifs there. The pole we're talking about will fall toward the road. ABB was showing their new high impedance fault algorithm at the last relay conference. Dry asphalt would not draw enough current to detect. Better hope the conductors short together on the way down, and then stay together throughout the reclose cycle.

 

[jghrist]

Actually, the pole won't fall. A section from the ground line to 20 feet above the ground will break away. If the adjacent poles hold, I figure that top section with conductors and crossarms intact will swing out to the center of the road in line with the adjacent poles. There will be a lot of slack because the distance between adjacent poles is less than the sum of the two original spans. The pole top with intact energized conductors will probably hang a couple feet off the middle of the roadway.
Of course, they will be swinging wildly back and forth across the road. Doesn't sound too safe to me.

 

[mc5w]

What Ohio Edison has been doing with 45 degree curves in roads is to put the pole at the middle of the curve on the inside of the curve. The 2 adjacent poles are placed on the outside of the curve. This solves both the guying problem and the motor vehicle collision problem.

Mike Cole mc5w at earthlink dot net

 

[stevenal]

So our motorist enters the corner too fast, but manages to hang on most of the way through before the rubber finally breaks free and he crashes into the non-breakaway adjacent pole. ACtrafficengr, how many of these collisions occur right in the middle? Are they so wasted they kept the wheel straight?

 

[ACtrafficengr]

I don't know of anyone has studied where along a curve run-off-road crashes are most likely. I'd assume that the danger zone would start froma point in line with the approach, and extend to a point past the end of the curve. If it's a sudden curvehidden by a hillcrest, for example, I'd expect more crashes nearthebeginning of the curve than other wise. If it is insufficiently banked, you may have more towards the end of the curve, as drivers enter at a speed that seems reasonable, and then find they are going too fast.

I'm dealing with two locations. The first is a sharp (100 ft radius) left curve at the bottom of a hill. Let's call the pole at the beginning of the curve pole #1. Pole # 2 is in line with apporaching traffic, and pole 3# is at the end of the curve. In three run-off road crashes in 15 months, pole # 2 was hit twice, and pole # 3 was missed by inches.

The other location has a pole that's been hit twice, even though it's on the inside of the curve. Go figure.

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"Come to think of it, there are already a million monkeys typing on a million typewriters, and the Usenet is NOTHING like Shakespeare.

Blair Houghton

 

[vtpower]

We are always careful of pole locations in "crash zones" but for the most part we push the pole far enough in the road Right of way to be behind a guard rail. I guess I am curious if you are not using guard rails in your situations? This seems as though it would greatly decrease the chance if hitting the pole.

 

[ACtrafficengr]

Too many driveways in the area to use guiderail.

Actually, guiderail is the second-to-last alternative when dealing with roadside hazards. The alternatives in order of preference are:
1. Remove the hazard (bury utilities or move them to alleys - both very expensive)
2. Relocate it to a location where it is less likely to get hit (probably the best option for utility poles where feasible)
3. Modify it so that it is less dangerous if it does get hit (breakaway poles)
4. Shield traffic from the hazard with guiderail
5. Warning signs or delineators to keep people away from it. (PA DOT is studying the effects of 4" wide reflective bands on utility poles. - a cheap measure that may be cost effective)

Also, most of our roadways are "three-rod roads." Our right-of-way is 49'6" wide. Even If we use rail, there isn't always enough

You might be interensted in a new Transportation Research Board report - A Guide for Reducing Collisions Involving Utility Poles, at

http://safety.transportation.org/guides.aspx?cid=31

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"Come to think of it, there are already a million monkeys typing on a million typewriters, and the Usenet is NOTHING like Shakespeare.

Blair Houghton