Vehicle impact attenuation speed. 2

[FishScreener]

I work for the Forest Service, and am working on developing plans to install new signage on approximately 45-miles of unpaved Forest road.

The speed limit is posted at 35-mph. There are about ten of the old style Forest Facility signs, that are installed in raised mortared rock platforms.

Does anyone know where I can find the design standards that detail: the distance from the travelway, and speed at which we need to worry about impact attenuation?

 

[Ltwine]

Local DOT or USDOT should be the place to visit.

 

[FishScreener]

I tried a google search for "fhwa vehicle attenuation", and didn't get anything that made sense.

I was hoping someone who does enough geometric design, or traffic engineering might know offhand which standards apply so I could narrow down the search.

 

[bridgebuster]

Download the MUTCD - Manual of Uniform Traffic Control Devices - Go to Page 43 (Section 2A.19 - Lateral Offset)
- it has guidelines for placing signs based on the site conditions.

http://mutcd.fhwa.dot.gov/

 

[bridgebuster]

I forgot to add that you should use a break-away post in the event of impact. Any state DOT website would have a standard drawing for mounting signs.

Good luck.

 

[FishScreener]

I'm kind of stuck between the rock and a hard place.

I'm trying to design to current standards, which require the breakaway bases. The Rangers want to keep the historic Forest Service Bases, which are raised pads made mortared rock. And admittedly they are attractive.

They are also hazards.

I was hoping that with the reduced speed they might be acceptable. But, the MUTCD doesn't seem to have exceptions. However, I remember one of the traffic engineers I used to work with, had a table that gave offset distances, to objects and posted speeds, that allowed non-breakaway objects in the right of way.

I just can't remember which reference book it was in.

 

[bridgebuster]

The book you're think of is the AASHTO Green Book, which states:

Horizontal Clearance to Obstructions
A clear zone of 2 to 3 m [7 to 10 ft] or more from the edge of the traveled way, appropriately graded with relatively flat slopes and rounded cross-sectional design, is desirable. An exception may be made where guardrail protection is provided. The recovery area should be clear of all unyielding objects such as trees, sign supports, utility poles, light poles, and any other fixed objects that might severely damage an out-of-control vehicle.

To the extent practical, where another highway or railroad passes over, the structure should be designed so that the pier or abutment supports have lateral clearance as great as the clear roadside area on the approach roadway. For further information on providing roadside lateral clearance, see the AASHTO Roadside Design Guide.Where it is not practical to carry the full-width approach roadway across an overpass or other bridge, an appropriately transitioned roadside barrier should be provided. At selected locations, such as the outside of a sharp curve, a broader recovery area with greater horizontal
clearances should be provided to any roadside obstruction."

My Roadside Design Guide seems to have grown legs.

 

[bridgebuster]

The attached should help.

 

[dik]

Thanks BB... good link.

 

[bridgebuster]

your welcome

 

[Drumchaser]

12 ft. from edge of travelway (not including shldr.) to edge of sign (crashworthy sign)
breakaway mount.

7-9 ft. sign height from edge of (elevation) travelway to bottom of sign.

 

[ACtrafficengr]

Bridgebuster's extract comes from the Roadside Design Guide. To get your own, go to:

https://bookstore.transportation.org/item_details.aspx?ID=148

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[civilman72]

AC got it right - you should be referencing the Roadside Design Guide (RDG). There is a chart in there that I'm willing to bet is the chart you mentioned with offset distances and speeds.

Drumchaser's statement of 12' offset is too generalized. It can vary depending on conditions such as horizontal alignment and road sideslopes, which is better described in the RDG.

Keep in mind that engineering judgement plays a large part in determining if impact attenuation is applicable. Just because there is a potential safety hazard in the "clear zone" it does not automatically need impact attenuation, particularly if there are no known history of vehicle collisions with the hazard. Don't forget that items such as guardrail are also traffic hazards and their installation can often decrease overall roadside safety. For instaance, installing 300 feet of guardrail 10' off the edge of road to protect a single sign post that is 20' off the side of road (but technically in the clear zone) creates a large roadside object that is much more likely to be hit by a vehicle than a single post that is 20' off of the road.

So, use the "requirements" within the RDG with caution, and be prepared to use some common sense.

 

[FishScreener]

Thanks. The existing signs, were designed and the bases installed by the CCC work crews in the mid to late, 1930's. They are considered, historical, and a real pain to modify.

Plus, in the 80 or so years they have been there, none of them have been hit from the roadway. There have been a few low speed impacts from tourists backing travel trailers, though.

 

[civilman72]

If these signs have been in place since the 30s, and have not caused any significant safety hazards in the past, then impact attenuation is probably not necessary.

 

[Drumchaser]

My apologies....I misinterpretted the OP to state that new signage was to be installed. (new signs and mounts)

The "mortared rock platforms" evidently would be an obstruction and not crashworthy.