Need Guidance On Intersection Markings

[joengr]

Good Morning,

I have attached a picture of an intersection with a proposed line extension for turning.

I know you can use a white line extension like this if you have a dedicated left turn lane. Can you also use a white line extension with a combined-use lane like in the picture? If so, what dictates the radius of the dashed line segment? What is the minimum spacing between the opposing turns?

I've tried looking in MUTCD as well as AASHTO but cannot find the answers to my questions. Any advice on this would be appreciated.

 

[ACtrafficengr]

I didn't see anything covering it in the MUTCD either. However, my first reaction is don't do it. I think you might confuse the through driver.

With the lane designations shown, I would go with split phasing for the side road. You might be able to overlap the left turns if you provide the extension lines and change to L/TR lane designation.

You should repeat your question on the MUTCD forum:

http://knowledge.fhwa.dot.gov/cops/opspublic.nsf/home?openform

"...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

 

[bimr]

The only time that I have seen that type of dashed line is between a double turning lane.

 

[Debaser]

I agree with ACtrafficengr in that any lines which drivers have to cross perpendicularly, have the potential to confuse. We're conditioned to stop or give way at such markings.

Having said that, if traffic is able to go ahead and left from the same lane then those markings 'blocking' ahead traffic will get scrubbed out pretty quickly anyway. However that's not the way to approach this problem.


Are those markings guiding drivers round the turn similar to others on the network in terms of length and gap? Are there any alternatives to this line which are totally dissimilar? (Occasionally we use a marking colloquially known as 'elephant's feet' which is approx. 300mm x 300mm squares at say 1.5 m spacing, this marking isn't prescribed for national use but is used at a Local Authority's discretion).

 

[joengr]

For the dashed white lines I was just going to go with the MUTCD measurements of 4 inches wide and 2 feet long with 6 foot gaps.

What if I changed the lane designation? One lane would be a specified left turn lane and the other would be designated for through traffic and right turns. I would then add a 'left turn yield on green' sign. What are your thoughts?

The problem with this intersection is its odd alignment and the wide 'dead space' in the middle. Currently there are no guidance lines so every time a driver enters the intersection they panic. When two cars from opposite directions are both trying to make left turns, there is massive confusion. Some drivers want to cut in front of the on coming driver and stomp on the accelerator. Some want to wait and go behind the other driver and end up blocking the intersection. With the current ambiguity and frequency of accidents at this location I am under the impression that anything will be an improvement.

Thank you for your help on this subject. This is the third traffic issue I have ever dealt with and feel like I am stumbling in this area. I am missing anything? Are there other guides or better guides beside MUTCD or AASHTO?

 

[ACtrafficengr]

Unless it would totally blow the L.O.S. out of the water, I would go with switching the lane designation, putting up left turn arrows and providing a dedicated left turn phase, or split the northbound and southbound approaches into separate phases.

You could get a copy of the ITE Traffic Control Device Handbook. It was published in 2001, so some of the info in it might be a bit of of date. It does provide a lot of guidance that should be in the MUTCD but isn't.

You could also ask for advice from your state's LTAP center:

http://www.ltapt2.org/centers/

Their job is providing technical advice and training to municipalities. They won't design it for you - it wouldn't be fair for FHWA-subsidized programs to compete with consultants - but they will try to point you in the right direction.

"...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

 

[eea]

I've seen these skip dashes applied, but on the inside of the left turn movement rather than the outside. Its application seems to be a District preference rather than a statewide policy in Illinois. 8” wide skip dash, 2’ long with a 6’ gap, as already mentioned. Personally, I like it.

Check clearance between opposing movements in the AASHTO Policy on Geometric Design of Highways and Streets aka "Green Book". Start with a review of the At-Grade intersections chapter. This is going to be a function of your designated design vehicle, as prescribed by your DOT and roadway classification.