Dual Left Turn lanes - Beginning Taper to one lane

[cgarlick]

We have a condition where we have dual left turn lanes going into the intersection and am looking for guidance on when to begin to taper the two lanes into one lane.

We looked through our state's design guide and also AASHTO design guide and did not find any guidance.

One initial thought was to begin the taper at the point where the vehicles reach the posted speed starting at the beginning of the tangent segment.

 

[cgarlick]

Actually the last sentenance needs some clarification. We would start to measure how long it would take for cars starting at a low speed while turning the corning(say 5 MPH) and determine how long it would take for them to reach the posted speed and THEN begin the taper.

Your thoughts and experiences are welcome as we have not found much guidance on this issue.

 

[ACtrafficengr]

Perhaps decision sight distance would apply?

What's your design speed? Is there a two lane through movement onto this leg of the intersection?

 

[cgarlick]

I did not consider decision sight distance.

The design speed on both legs is 45 MPH. There is a two lane segment approaching the intersection for this movement.

 

I checked Illinois DOT standards and they don't have any taper guidelines for acceleration lanes like this either. If sight-distance doesn't work as a guide, I would use the standard approach taper rate for your state. In Illinois, for a 45mph road, the taper rate is 35:1. I would probably put a 100' tangent on first.

Hope this helps,

Rob

 

[jcfree]

As far as the taper length goes, I would suggest you not go below the minumum allowable in the MUTCD for a merging taper for temporary traffic control zones; i.e., L = W * S for speeds of 45 mph or greater, or L = W * S^2 / 60 for speeds of 40 mph or less. I haven't seen any hard criteria for the length of lane downstream from the intersection before you begin the lane reduction, but I seem to recall the old Jack Leisch methodologies recommended something like 200 to 300 ft., but I'm not certain of this distance. You also might see if there's some guidance in the Highway Capacity Manual, based on merging speed, etc.