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minimum road width based on turning radius

minimum road width based on turning radius

minimum road width based on turning radius

I always thought 20' lane width is good enough but I'll be using smaller lane width this time.

This drawing is confusing as far as minimum road width required for 18-wheeler turning 90 degrees.
If I follow path of right - rear wheel, it looks like I need more than 20' road width?
This doesn't make sense.

RE: minimum road width based on turning radius

it was taken from AASHTO but question is am i interpreting this right?

for 18-wheeler to turn 90 degrees, I need 45' turning radius and more than 20' road width?

RE: minimum road width based on turning radius

That sounds reasonable; I'm dead positive that it's impossible for semi to turn directly into a single lane alley. Conversely, try to think of all of the semi right turns onto 4-lane roads. The semi will invariably go into lane 1 of the cross street, particularly since the rear wheels have to clear the curb. Lane 1, here, is the centermost lane. I've been in the left turn lane many times, dreading the semi coming at me and finally turning right.

I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: minimum road width based on turning radius

Turning trucks don't need wide roads. Corner radius is more important than road width. Once it completes the turn, 10 ft or 11 ft lanes are fine. If you don't want the truck to encroach on the oncoming lane, you'll need a large corner radius, or one of the alternates in the Green Book like a three-centered curve or curve with tapers.

It really depends on the context. On a high speed rural road, a larger radius will allow the truck to complete the turn quickly and reduce the rear-end collision risk. In a city center, a tighter radius is often desirable.

RE: minimum road width based on turning radius


the swept path width, doesn't mean the width of the road required? the path of right rear wheel is not something I should be concerned about.

to confirm 20' wide road is more than enough for wb50 truck to make 90 degree turn. as you said only around 12' road width is required

RE: minimum road width based on turning radius

My apologies if we are misunderstanding each other. When you say road width, I think of the road segments between intersections. At the intersection, yes, you'll need to provide more width by using a curve connecting the two roadways.

Do you have the complete Policy on Geometric Design of Highways and Streets (Green Book) that your turning template came from? If so, turn to the section on Design for Specific Conditions (Right Angle Turns) in Chapter 9.

Or is this a 90o bend in a roadway? In that case, turn to the section on offtracking and curve widening in Chapter 2. That describes how much added width you'll need, based on the vehicle type and centerline curve radius.

RE: minimum road width based on turning radius


this is what I mean. is 20' road good here?
yes I have green book and will look at chapter 2 and 9

RE: minimum road width based on turning radius

Is this a one-way road, or low enough volume that you won't need to worry about two-way traffic?

You'll need some curve widening. You'll need more room on the inside for the back wheel off-tracking, and wheels right at the edge of pavement will cause edge cracking. The minimum I'd use would be the "minimum edge-of-traveled way design" from Chapter 9. It's Exhibit 9-24, page 600 in the 2004 edition.

If you use a larger radius, you can use a narrower roadway. If you use 75' centerline radius, a 25' pavement width should be enough for most drivers in a WB-50.

RE: minimum road width based on turning radius


This is a low volume traffic inside private industrial site.

I just saw this in client specs. So you said, I don't need 75' centerline turning radius if I use curve widening.

RE: minimum road width based on turning radius

If the client wants the 75' radius, go with that. It will be easier to build and use less material.

RE: minimum road width based on turning radius

turns out we have this cool software called autoturn. now I can play with road width, inside and outside turning radius.

RE: minimum road width based on turning radius

Keep the truck path at least 3 feet from the edge of pavement. While I've seen some professional drivers perform impressive feats of vehicle placement, I've also seen some rather boneheaded maneuvers. Also, you'll get edge cracking with trucks that close to the edge.

RE: minimum road width based on turning radius

Assuming this is in the States, did you intend to place the semi in the opposing lane of traffic at the start of its maneuver? If that is a side road, then the truck should start closer to the curb / EOP and would benefit from the 3-centered curve already mentioned. I usually offset the center line by 2' to get the edge of the vehicle (driver "shy" line) plus another 4' for the center of the truck and fillet the two offset tangents with the minimum turning radii of the WB unit (75' to 80' 'ish).

There is also a shareware / free edition of turning movement software, see http://www.glamsen.se/CadTools.htm. The author, Lars has a version for AutoCAD and Microstation. Pretty similar to the old v2.0 of AutoTurn. Forward movements only.

RE: minimum road width based on turning radius

I think that is from Tom Haws' website. I've spoken with Tom a few times over the years, great guy who's willing to help and has some great utilities. I've tried both CadTools and Turn.LSP, but find the GUI of CadTools a little more straight forward. That being said, Lars is in Europe so Metric is the default scale. I have had the unit setting get weird on me, plotting at 1/3.28 even when I've thought my units where set to Feet not Metric. I think I saved a custom template for a WB-65 and re-inputted each dimension in English as a workaround.

RE: minimum road width based on turning radius


The cad file was developed by myself about 25 years ago from hand drawn drawings and the information came from a publication "How Big is a Truck and How Large does it Turn" or something similar. I've used the cad drawings for several projects.


RE: minimum road width based on turning radius


I'm sorry for that. Tom's site http://www.hawsedc.com/gnu/turn.php does reference Turn.Lsp as OpenSource and as developed with a co-author.


RE: minimum road width based on turning radius

Not a problem... the hand drawing was done originally for the Cornwall Centre in Regina for an underground loading dock. Worked it out on paper, then once the columns were located (on paper), hired a rig to check that it worked. Worked great... It's not a lisp routine...

Others can use the Lisp link, thanks...


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