Is there a standard that transportation engineers use for the number of vehicles a road should carry daily for it to be widened to four, six, eight lanes, etc? Curious because I've noticed that some states seem to be unnecessarily widening their interstates to six lanes in rural areas, and others refusing to do so in places that need it.

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    $\begingroup$ Not sure, but I remember a radio interview in London where they interviewed the person responsible for traffic flow and asked him about how he made / ratified the decisions for lights / roundabouts etc. The stunned silence as he admitted he did not have a driving license and could not drive .... lol Mind you this was back late 80's $\endgroup$
    – Solar Mike
    Aug 20, 2017 at 20:59
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    $\begingroup$ What you have observed may be due to "political" reasons, not engineering ones. There's a adage "the squeaky wheel always gets the oil". It may be the result of intense lobbying, either in the open or behind the scenes. Then again, the widening may be due to advanced forward planning & the lack of development may be due to lack of funding or even lack of forward planning. $\endgroup$
    – Fred
    Aug 21, 2017 at 3:23
  • $\begingroup$ I know some of it has to do with the ways states fund transportation. I was specifically thinking of my home state of Tennessee. In TN, they don't buy bonds, and rely on savings from fuel tax revenue to fund projects, and are unable to accomplish some projects that they would like to. In several neighboring states, however, they use GARVEEs, and that's where I've noticed unnecessary widening in rural areas. Much of that certainly is probably political. $\endgroup$
    – Sagierian
    Sep 14, 2017 at 2:42

1 Answer 1


"Vehicles per day" is a pretty bad metric to use. Instead you should look at rush hour and see how many vehicles are trying to get through.

Every lane can carry 1800 vehicles each hour on a freeway. In residential areas that is reduced to 1500. With traffic lights and priority based intersections it depends on how the lights are regulated and how much crossing traffic there is.

Having said that: the point where you are stuck in a jam is not the point where you should add a lane.

Instead you should try and find the actual bottleneck and improve that. This bottleneck is where you can start going the speed limit again. This may be an on-ramp where the traffic added saturates the lanes that are there, or an off-ramp where everyone wants to get off and that is saturating the interchange.

However just improving that bottleneck will add traffic downstream and probably create a new jam.

  • $\begingroup$ Theoretically we can deal with the bottleneck by increasing speed at that point - conservation of flow :) Of course, it does assume that all the cars can achieve the necessary speed .... Don't shoot me! $\endgroup$
    – Solar Mike
    Aug 21, 2017 at 8:51
  • $\begingroup$ @SolarMike no you cannot increase capacity by increasing speed. Higher speed means keeping more distance between cars, the 2 second rule is there for a reason. $\endgroup$ Aug 21, 2017 at 9:01
  • $\begingroup$ Two points : one I said "theoretically" and if you change the diameter of a pipe the speed changes if all else stays the same and second : do ALL drivers obey the two second rule .... given what one can see standing on a bridge over a motorway and the number of accidents I wiuld suggest NOT .... $\endgroup$
    – Solar Mike
    Aug 21, 2017 at 9:10
  • $\begingroup$ @SolarMike particle flow and traffic have a few parallels but that is not one of them. $\endgroup$ Aug 21, 2017 at 9:12
  • $\begingroup$ So, in your opinion do some, most, any or all drivers use , apply or understand the two second rule??? $\endgroup$
    – Solar Mike
    Aug 21, 2017 at 9:14

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