Thorough (bred) AADT

[intelati49]

What is the aadt of a fourlane freeway that you would need to expand the road to six lanes? Assuming that construction and land is free, when would you need to expand to eight?

In Springfield, MO, MODOT expanded US65 when traffic counts were at 63,000 daily. What would your REALISTIC (for lack of better terms) target (?) AADT be?

What is the aadt of a fourlane EXPRESSWAY that you would need to expand the road to six lanes? Assuming that construction and land is free, when would you need to expand to eight?

In Springfield, MO, MODOT expanded US160 when traffic counts were at 45,000 daily. What would your REALISTIC target AADT be?

And finally do you have an idea of the capacity of a one lane loop ramp (Freeflow exiting with no weaving.) Two lane loop? Or a one lane flyover? Two or three lane flyover?

[Duke87]

There's no one answer. Depends on how much of the traffic is trucks, how much the traffic peaks during rush hour, and what LOS you want to provide.

[intelati49]

There's no one answer. Depends on how much of the traffic is trucks, how much the traffic peaks during rush hour, and what LOS you want to provide.

*facepalm* TRUCK ADT 7%, PEAK HR 11%, and urban target los is "E"

[NE2]

This has the feel of someone asking for homework help

[intelati49]

This has the feel of someone asking for homework help

I wish. one more year.

[Alps]

Definitely depends on the area. In NJ, we force-feed 110,000 AADT through four lanes, and in NY 250,000+ are shoehorned into six lanes. That said, ideally you'd want to go from four to six somewhere around 50,000 AADT on a freeway (you're close to 5,000 in the peak hour, about 1,600 per lane in the peak direction), maybe 35,000 AADT on an expressway. For an urban area out west with enough room, like Springfield, 63,000 and 45,000 seem perfectly reasonable. For an urban area in the northeast with no room, you'd probably push to the limits - 75,000 and 50,000, or even more.

[J N Winkler]

This thread reminds me of an attempt the Road Research Laboratory made in the 1950's to come up with an economic definition of what was then called "practical capacity."  The Laboratory researchers postulated that, for a given width of typical section, there was an AADT value above which the delays to traffic would be sufficient to amortize the capital cost of a widening, and that AADT value was thus the "economic capacity" of the road.  The problem with this approach, of course, is that land acquisition and construction costs are unavoidably part of the capital cost of widening, so the economic capacities of different roads would correspond to different LOS when the underlying costs of widening varied widely.

[intelati49]

This thread reminds me of an attempt the Road Research Laboratory made in the 1950's to come up with an economic definition of what was then called "practical capacity."  The Laboratory researchers postulated that, for a given width of typical section, there was an AADT value above which the delays to traffic would be sufficient to amortize the capital cost of a widening, and that AADT value was thus the "economic capacity" of the road.  The problem with this approach, of course, is that land acquisition and construction costs are unavoidably part of the capital cost of widening, so the economic capacities of different roads would correspond to different LOS when the underlying costs of widening varied widely.

Huh???

[Scott5114]

This thread reminds me of an attempt the Road Research Laboratory made in the 1950's to come up with an economic definition of what was then called "practical capacity."  The Laboratory researchers postulated that, for a given width of typical section, there was an AADT value above which the delays to traffic would be sufficient to amortize the capital cost of a widening, and that AADT value was thus the "economic capacity" of the road.  The problem with this approach, of course, is that land acquisition and construction costs are unavoidably part of the capital cost of widening, so the economic capacities of different roads would correspond to different LOS when the underlying costs of widening varied widely.

Huh???

He's saying this thread reminds him of an attempt the Road Research Laboratory made in the 1950's to come up with an economic definition of what was then called "practical capacity."

[J N Winkler]

Let me illustrate it with an example.  Let us say you have a four-lane freeway which carries an AADT of, say, 70,000, and you are trying to decide whether it is worth widening it to six lanes.  You start with the premise that the time drivers save on the widened road pays for the cost of the widening over time (time is money, so you just multiply the time savings by a nominal money value of time, such as an estimated hourly wage).

Let us then say that the construction cost of widening to six lanes is fixed, whether it takes place in an urban or rural area.  In the case of the rural location, the decision to widen is a no-brainer:  land acquisition cost is typically a small fraction (say 10%) of construction cost in rural areas.  In fact, unless construction is really expensive, you probably come out ahead if you make the decision to widen at a much lower AADT, say 50,000.

In the urban area the decision is more difficult.  Frequently the construction cost is 10% of the land acquisition cost, not the other way around.  The time savings have to be correspondingly high in order to justify a capital cost which is around ten times higher than the rural widening.  You might not decide to widen at 70,000 AADT; you might instead defer the decision until AADT reaches, say, 100,000 with some significant peak hour spreading.  When you make a decision like that, you are in effect accepting a lower level of service (LOS) on the freeway for a greater proportion of the day than in the rural case.

This is a drastically oversimplified example, but it illustrates the basic concept.

What the Laboratory was trying to do was to link the AADT "trigger point" for widening (and other types of improvement) to the break-even point where time savings exactly counterbalance capital cost of construction and land acquisition.  Their idea was that the trigger point was effectively the practical capacity of the road.  My point was that this leads to radically different values for capacity in urban and rural areas.  (In the US we finesse the urban/rural distinction by having separate design-year LOS for each context.  In California, for example, the old Caltrans standard was LOS B in rural areas and LOS D in urban areas.)

[Alps]

I've done work on a "practical capacity" of sorts for work zones, and what I was finding is that the LOS D/E threshold is a good approximation of when a roadway's "freedom" is essentially used up. Although it could theoretically hold more, above that threshold is where minor incidents (maybe not every day, but at least once a week) cause tie-ups. Below that threshold, traffic flows fairly well most days. Rather than get into too much technicality here, that leads to roughly 1,900 to 2,100 vehicles per hour per lane for most freeways (in each direction), or an AADT of about 40,000 vehicles per lane pair (1 each way). Widen a four-lane freeway when it gets above 80,000 AADT (or somewhere in the 70-90 range), widen a six-lane freeway somewhere in the 100-130 range, etc. This works best in urban areas; in rural areas 80,000 AADT would already be filling up a six-lane freeway.