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Banking transitions and longitudinal grade.

Banking transitions and longitudinal grade.

Banking transitions and longitudinal grade.

(OP)
I'm working on a design for a low volume rural road (ADT=330). It's a "legacy road" that was probably never engineered. The reported safety history is favorable, so, I don't want to go overboard and reconstruct the whole road.

There are a few sites where the end runout/begin runoff point is at a location with little longitudinal grade. I can straighten a curve at the worst site and remove the need for banking, but I'm curious what other solutions people have come up with.

I'm also curious if anyone has written a book of solutions to  common design problems. The Green Book often says "use engineering judgement" but is lacking in guidance. The AASHTO LVRR book is a bit short in this regard, too.

     "...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

RE: Banking transitions and longitudinal grade.

Does a road with this little traffic really need superelevation?  I'm guessing that this a local road, and 95% of drivers are familiar with the existing geometry and tight-spots.  As the LVRR book states, roads such as these do not necessarily require the same level of design (superelevation, roadside safety zones, etc.)as other higher-volume roads.  In many circumstances when dealing with local traffic, it may be easier and more cost effective to provide signage than to reconstruct the road.

If the safety history is favorable, why are you proposing to change the geometry in these locations?  Is your concern with the lack longitudinal grade due to drainage/standing water?  Since this road is supelevated, I'm assuming it's a paved surface?

I too have been looking for a book or website on common road design problems (particularly for existing road improvements) and have yet to find one.

 

RE: Banking transitions and longitudinal grade.

(OP)
Thanks.

My initial approach was going to be keep the existing cross slopes, as per the LVRR guidelines, but then I saw how badly out of shape the road is. Most of the curves are dead flat on the outside lane.

We have some pavement segregation problems with the existing pavement, so more than a simple overlay is called for. If we reclaim the pavement, we can play with the cross-slope a bit more.

So, I was thinking of using 4% e-max. Would you consider reverse crowns on the curves? It's a 55 mph roadway.

While only three reported crashes have occurred in the last 10 years, the locals told our survey crew that there have been many unreported off-road excursions as well. Our surveyors also reported standing water on the pavement in some of these areas.

     "...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

RE: Banking transitions and longitudinal grade.

I would use reverse crowns (typical crown) if the centerline radius met the AASHTO requirements for no superelevation, but for a 55mph road, that's a radius of about 8,500', which is probably not feasible.

Also, using 4% e-max seems a little too conservative - is this a local road standard requirement?

The combination of standing water within a substandard horizontal curve would concern me, and your idea of increasing the superelevation (using reclaimed asphalt) seems like a prudent idea, particularly if the standing water is accelerating the pavement segregation.

Have you read the AASHTO "A Guide for Achieving Flexibility in Highway Design - 2004?"  While it does not provide site specific design alternatives/examples, it does discuss general mitigation measures to consider when the design engineer is proposing to use substandard road design parameters.

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