Criteria for a 6-lane interstate?

[lamsalfl]

What is the criteria of average annual daily traffic for an Interstate to be 6 laned?  Is there a rule of thumb? 

I'm tooling around on LA's traffic count maps trying to come up with some sort of ballpark number.  As for my information of what Louisiana interstates are "eligible" I'm using 46,000.  Why?  Money has been secured to widen I-12 to Walker (Exit 15).  On the maps, the I-12 traffic just west of the Walker interchange was 46,494 in 2009.  Based on this assumption that that figure is good enough for a six-lane, I've determined almost all of I-12 is in need of six-laning.  (Not surprising as every time I'm on it traffic is pretty good.)  Also, I-10 from BR to NO is in a similar situation. 

http://www.dotd.la.gov/highways/tatv/default.asp

[froggie]

There's no specific criteria relating to JUST AADT.  Using the Highway Capacity Manual, AADT is one of the factors that go into LOS calculation, but there are several others, including peak-hour factor, width of lanes & clear zones, percentage of truck traffic, etc.  Then there's the urban/suburban/rural area bit and preferences of the local DOT.  Some consider LOS D acceptable for rural areas.  Others don't.

[lamsalfl]

I also suspect truck traffic and politicians touting "economic development" play a role in this too.

[Revive 755]

Looking over the volumes for I-70 across Missouri in 2000 (first year I remember hearing anything about of redoing this route due), I-70 across Indiana, and 2009 volumes for I-57 between I-64 and I-24 in Illinois, I would say if going by ADT alone 30,000 is a good cut off for four versus six lanes.

[lamsalfl]

yeah to try and get a good number, I was researching upcoming widening projects and looking at their latest traffic volumes. 

[kharvey10]

well I-270 from I-255 to Lilac is still at 4 lanes and approaching 60k AADT in spots, not to mention 20% to 25% percent of that is truck traffic and its known for bad accidents.  Its at LOS D from what the EWGateway documents got (surprised its not an LOS E or F). 

The big barrier that is keeping it at 4 lanes is the canal and river bridges, both which would need to be replaced.  IDiOT is only planning on replacing the canal bridges but thats at least a couple years away.

[6a]

When the interstates were designed, a city had to have 1,000,000 people to be eligible for six lanes.  My, how far we've come.

[Brandon]

When the interstates were designed, a city had to have 1,000,000 people to be eligible for six lanes.  My, how far we've come.

My, how short-sighted we were.

[roadfro]

When the interstates were designed, a city had to have 1,000,000 people to be eligible for six lanes.  My, how far we've come.

I somewhat doubt that this was a firm criteria. The amount of traffic projected to use the facility according to various factors (such as those froggie stated) would be more relevant to whether an Interstate Highway would be built with three lanes each way or two. Certainly, projected traffic volumes could be reasonably assumed to be higher in a city with a greater population, but there's other factors at play.

[froggie]

I would say if going by ADT alone 30,000 is a good cut off for four versus six lanes.

Not really.  With 'typical' values, a 30K ADT roughly corresponds to LOS C.  Desireable in a rural area, but definitely under-capacity overall or in an urban area.

[english si]

I think the cut-off in England is 60,000+ nowadays - 30k with 4 lanes is far from problematic - the problem, if any, is low-capacity junctions or weaving due to too small junction spacing.

Of course, England is the population of California and Illinois (51 million ish) in the area about the size of Lousiana or Mississippi (50,000 sq mi and change), so the population density and lack of space means we need to get more out of our road network than most of the US - the North East excepted.

[deathtopumpkins]

well I-270 from I-255 to Lilac is still at 4 lanes and approaching 60k AADT in spots, not to mention 20% to 25% percent of that is truck traffic and its known for bad accidents.  Its at LOS D from what the EWGateway documents got (surprised its not an LOS E or F).

60k on a four-lane? That's nothing. In 2009, traffic on I-64 betwen Hampton and Norfolk, VA hit 87,000 AADT accordoing to VDOT's counts [pdf], and according to the following HRBT study measured in 2008 it was 92,800. As for LOS, the HRBT Expansion Feasibility Study (released Nov 2010) [pdf] cites a projected LOS of F for 2018. Now this is a special case, as it is a bridge-tunnel and thus not easily expanded (built 1956, expanded 1976), I just felt like pointing out that you aren't alone in your grief. Additionally, I doubt this is representative of Virginia standards.

[J N Winkler]

I somewhat doubt that this was a firm criterion. The amount of traffic projected to use the facility according to various factors (such as those froggie stated) would be more relevant to whether an Interstate Highway would be built with three lanes each way or two. Certainly, projected traffic volumes could be reasonably assumed to be higher in a city with a greater population, but there's other factors at play.

It could have meant just that a special case had to be made for initial provision of six lanes when the population was less than 1 million.

Capacity analysis was very rudimentary at the time the first few Interstates were being planned.  The first edition of the Highway Capacity Manual was not even published until 1950, and it used the concept of "practical capacity" instead of classification by letter-grade Level of Service.  The LOS concept was not introduced until the second edition of the HCM, which was published in 1965.  Some years later (1967 or 1969), Caltrans came out with a capacity primer which explained that its general intent was to design for LOS B in rural areas and LOS D in urban areas.

Prior to publication of the first edition of HCM, there was some consideration of capacity and demand in planning new roads, but it tended to take the form of general rules of thumb.  For example, at one time 4000 VPD was a popular warrant for widening to four-lane divided.  In Britain, designers were urged (in a 1935 circular dealing with the Restriction of Ribbon Development Act) to plan for dual carriageways on any road anticipated to carry more than 400 VPH during the peak hour at some indefinite time in the future.

These crude rules of thumb were used on the (tacit) understanding that choice of how many lanes to provide, how high a design speed to cater for, steepest grade, etc. would be made through an iterative planning process.  The designer would choose a particular combination of lane count and design speed, look at how that affected the cost, and then go back and fiddle with various parameters until the best result was found which would fit the project budget while satisfying the purpose of the project.

[kharvey10]

Here is another 4 lane section targeted to be widened to 6 lanes; I-39/90 from Madison to the IL state line
http://www.tristatealliance.com/projects.html has more goodies from that area, some that are rather surprising

[Rupertus]

As far as Michigan is concerned, these are some rough minimum average daily traffic counts for 2009 according to MDOT along interstate stretches that are at least 3 lanes in each direction:

I-75 from Bay City to Flint: 52,000 (that minimum is between exits 136 and 144, where it is 4 lanes in each direction)
I-75 from Flint to Detroit: 40,000
I-75 from Detroit to the Ohio state line: 32,000
I-94 from Benton Harbor to the Indiana state line: 35,000
I-94 just west of Ann Arbor: 54,000
I-94 from Ann Arbor to Detroit: 79,000
I-94 through Metro Detroit: 104,000
I-96/I-69: 51,000
I-96 from Howell to Detroit: 46,000

Now, for some personal perspective, my drive to work in Ann Arbor is along US 23 from Washtenaw Ave. to I-94 (73,000) and I-94 to State St. (82,000), both of which are 2 lanes in each direction. Awesome.

[Chris]

The level of service drops significantly above 70 000 vehicles per day, taking an average truck share into account (10 - 15%). However, very high volumes are known to exist on 4-lane freeways. For instance nearly all of Tokyo's expressways have 4 lanes and most of them are loaded to 100 000 vehicles per day. The Netherlands was notorious for its large number of rural freeways with more than 80 000 vehicles on 4 lanes (making long-distance driving unpleasant). Luckily that has changed somewhat in recent years. The highest 4-lane count in the Netherlands is 110 000 vehicles per day. The highest 6-lane count is 180 000 vehicles per day (both in Amsterdam). These are hourly volumes that exceed the theoretical upper limit (3000 vehicles per hour per lane has been recorded).

[6a]

Now, for some personal perspective, my drive to work in Ann Arbor is along US 23 from Washtenaw Ave. to I-94 (73,000) and I-94 to State St. (82,000), both of which are 2 lanes in each direction. Awesome.

I-485 in Charlotte has a 4-lane stretch that clocks 120,000/day.  It's an absolute peach to drive, too.

[InterstateNG]

Now, for some personal perspective, my drive to work in Ann Arbor is along US 23 from Washtenaw Ave. to I-94 (73,000) and I-94 to State St. (82,000), both of which are 2 lanes in each direction. Awesome.

And it's going to be very difficult to expand capacity in those areas.

[ftballfan]

As far as Michigan is concerned, these are some rough minimum average daily traffic counts for 2009 according to MDOT along interstate stretches that are at least 3 lanes in each direction:

I-75 from Bay City to Flint: 52,000 (that minimum is between exits 136 and 144, where it is 4 lanes in each direction)
I-75 from Flint to Detroit: 40,000
I-75 from Detroit to the Ohio state line: 32,000
I-94 from Benton Harbor to the Indiana state line: 35,000
I-94 just west of Ann Arbor: 54,000
I-94 from Ann Arbor to Detroit: 79,000
I-94 through Metro Detroit: 104,000
I-96/I-69: 51,000
I-96 from Howell to Detroit: 46,000

Now, for some personal perspective, my drive to work in Ann Arbor is along US 23 from Washtenaw Ave. to I-94 (73,000) and I-94 to State St. (82,000), both of which are 2 lanes in each direction. Awesome.

I-196 is three lanes each way for a short section in Grand Rapids and unsigned I-296 is three lanes each way for its entire length.

[Rupertus]

I-196 is three lanes each way for a short section in Grand Rapids and unsigned I-296 is three lanes each way for its entire length.

Sure. If we want to talk about 3di stretches that are 3 lanes in each direction, then there are also parts of I-475 and I-496, if memory serves. I-696 is three or four lanes in each direction and is almost always a mess (125,000 at the bare minimum, but mostly above 150,000, and approaching 200,000 east of I-75). The portion of I-275 that duplexes with I-96 is four lanes each way and packed nearly all the time (over 167,000 per day), but once you get south of the airport on I-275 you'd be surprised at how little traffic there is. According to the MDOT map it gets down around 25,000 before dropping to 2 lanes each way.

[Alps]

I-95 and I-278 in NYC are three lanes each way (through lanes, at least) and well over 200K vehicles a day. Given that the capacity is about 6500-7000 vehicles per hour (accounting for trucks) in each direction, and that traffic jams knock that down to about 5000-5500, you're probably running at capacity about 8 hours a day and near it another 8. (8 * 12000 = 96000, 8 * 10000 = 80000, leaving about 40000 for the other 8 hours in both directions)