Question about highway traffic capacity

[Gnutella]

What's the minimum and maximum ideal daily traffic volume per two lanes on a road or highway? What I mean by the minimum is that expanding a highway would not have its expense justified if traffic volume is below it, and what I mean by the maximum is that expanding a highway would be absolutely necessary if traffic volume is above it.

If I had to guess, a road would need a minimum daily traffic volume of at least 10,000 vehicles to justify the expense of expanding it to four lanes, and a maximum daily traffic volume of 20,000 vehicles would make expansion to four lanes absolutely necessary. For six lanes, I'm guessing that the minimum would be 25,000 vehicles to justify it, and the maximum would be 40,000 vehicles to make expansion to six lanes absolutely necessary.

If anybody could tell me, I'd appreciate it. I've been looking though some traffic count maps that I've found, and I want to get an idea where road expansion is or is not necessary.

[froggie]

Depends on several factors besides just daily traffic counts.  Presence of traffic signals, presence and number of driveways or side streets, truck volumes, peak hour volumes, directional distribution (percentage of one direction versus the other), speed limit...all play a factor.

It also depends on the jurisdiction and how comfortable they are with queuing and platoons.  I've seen planning-level values anywhere from 5,000 to 15,000 vpd.

[johndoe]

Depends on several factors besides just daily traffic counts.  Presence of traffic signals, presence and number of driveways or side streets, truck volumes, peak hour volumes, directional distribution (percentage of one direction versus the other), speed limit...all play a factor.

It also depends on the jurisdiction and how comfortable they are with queuing and platoons.  I've seen planning-level values anywhere from 5,000 to 15,000 vpd.

I would add that a more accurate measure for what the op is discussing would be the peak HOURLY volume (as opposed to daily traffic).  Facilities are often designed for what goes on for two hours of the day; so it's "over-designed" the other 90% of the time.

[roadfro]

I imagine you'd also have to figure in that there would be a Level of Service (LOS) for the facility, given the various other factors. Road design would take into account the desirable LOS for the roadway at peak hourly flows...if the LOS is lower than desired, that could be what prompts expansion.

[Tom958]

Directional distribution can have a whole lot to do with it, too. I've seen freeways carrying 10,000 vpd per lane that were parking lots in the peak direction during rush hours and empty going the other way.

[cpzilliacus]

What's the minimum and maximum ideal daily traffic volume per two lanes on a road or highway? What I mean by the minimum is that expanding a highway would not have its expense justified if traffic volume is below it, and what I mean by the maximum is that expanding a highway would be absolutely necessary if traffic volume is above it.

If I had to guess, a road would need a minimum daily traffic volume of at least 10,000 vehicles to justify the expense of expanding it to four lanes, and a maximum daily traffic volume of 20,000 vehicles would make expansion to four lanes absolutely necessary. For six lanes, I'm guessing that the minimum would be 25,000 vehicles to justify it, and the maximum would be 40,000 vehicles to make expansion to six lanes absolutely necessary.

If anybody could tell me, I'd appreciate it. I've been looking though some traffic count maps that I've found, and I want to get an idea where road expansion is or is not necessary.

You cannot base investment decisions entirely on traffic count maps.

But to answer your question, the Highway Capacity Manual is the "Bible" that engineers and planners use to discuss and research such matters.

[cl94]

Going with what has already been said, don't base everything on the traffic count maps. It's all flow rate. If a highway is empty for 22 hours per day and jammed for 2, you design for those two hours. Typically, one designs for an average level of service at LOS C (relatively free-flowing) with peak at the high end of LOS D (near capacity, some minor speed reduction). Unless you're in a large urban area where highway expansion is near-impossible or cost-prohibitive, LOS E (at capacity, any incident causes large delays that brings it to LOS F (stop and go)) is a giant red flag to be avoided at all costs and, if it occurs, should be resolved. One typically begins to consider widening/other improvements at LOS D and draws up plans at E or worse, as such locations are deficient.

Of course, LOS F is unavoidable in certain locations (New York, New Jersey, etc.), but those are exceptions and, in those areas, one designs for an assumed LOS E and hopes for the best.

For many metro areas, the MPO has LOS maps on its website that at least show D-E or worse.