Safety effects of 2 to 3 lane conversions

[ACtrafficengr]

I'm looking for info on the safety effects of converting a two lane road to a three lane road within the same footprint, i.e. sacrificing the existing 6 ft. shoulders for the center turn lane. It's a suburban minor arterial, mostly residential with a few businesses.

I mostly want to know whether the crash reduction factor for the center turn lane would more than offset the loss of the shoulders, but I'd also be interested in effects on vehicle speeds or any other safety effects.

Thanks!

 

[racookpe1978]

Don't have the "official" highway department "studies" that the academics like getting paid to create, but in a suburban (low speed, irregular) road, accidents and failures can get off the road easily to parking lots or side roads - they're not in way of high-speed traffic as on an expressway.

With turn lane in the center, both through traffic people can go by unhindered by left-turners.

Curbs? Or gravel and grass on the sides?

 

[ACtrafficengr]

Thanks for replying.

It has paved shoulders with curb and a sidewalk on one side, concrete gutter on the other.

I'm asking because some people want the turn lane, and others want to designate the shoulders as bike lanes. It would be an important connecting link in a nascent bike network. There aren't any bikeable parallel streets. I'm trying to set aside my personal biases here and come up with the most rational argument for either one.

Looking at the crash modification factors at FHWA's CMF Clearinghouse, I'm not seeing a clear trend either way. There is no reported CMF for exactly what I'm looking for. The results for both adding shoulders and adding a center turn lane range from moderate decreases to slight increases in crashes.

Assuming the CMF for removing shoulders would be somewhat close to the reciprocal of the CMS for adding them, I suspect the result may be a wash.

 

[LittleInch]

I'm no highways engineer, but I can't see how a central turn lane would reduce crashes. The prevalent crash would normally be the vehicle doing the turning. It might actually increase crashes due to the issue of vehicle trying to exit the side road having to go a greater distance / difficult seeing past anyone trying to turn in, perhaps going half way across and then having to wait.

Speed of vehicles and reduction of slowdown / stop is your key issue but then also squeezes any cycle lane / cyclist.

Your CRF for the cyclists might be interesting though...

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