Thinking About Interstate Highway Multiplexes

[yanksfan6129]

I was, yes, thinking about interstate highway multiplexes, (specifically, looking at the I-55/I-74 multiplex in Bloomington, IL and Normal, IL) when I realized something.

Shouldn't multiplexed interstates carry the amount of lanes each multiplexed road carries going into the multiplexed combined?

Think about it. Traffic from two interstates combine, so even in rural areas, traffic going different places coming from different places combines. Wouldn't this theoretically mean double the traffic?

This is a very flawed theory, and I just want to see what other people think about this.

[Bryant5493]

Well, that makes sense. But sometimes right-of-way may be an issue.


Be well,

Bryant

[yanksfan6129]

In the case of I-55/I-74, it doesn't seem like it.

But regardless of right of way, it seems to me like they should have planned for extra traffic due to the multiplex.

[un1]

I don't know about that, because an interstate might carry 3,000 cars daily and merge with one that carries another 6,000 cars per day, I don't think that that would deserve a eight lane freeway, or even in that case a freeway. 

Although it would be awesome to have 12 (6 in each direction) lanes on the I-39, I-94 and I-90 stretch.

[corco]

I guess the way I look at it is that you're assuming that everybody on one route will never leave that route. To use the I-55 example, if I'm heading northbound on I-55 and I'm going to Indianapolis, I'll probably get on I-74 east, avoiding the multiplex altogether. Likewise, if I'm on I-74 east, I can certainly get onto I-55 north, also avoiding the multiplex.

If you look at the concurrency as two 3 way interesctions instead of a concurrency, then it works fine. If we added lanes at every single intersection, we'd have a 50 lane wide Interstate 80 somewhere

[rmsandw]

One thing about the I-55/74 multi-plex (or "West-Plex", as I wanted to call it when I was on the air in Bloomington radio), it was up until 5 years ago before the 3rd lane from the South Vets Interchange to Raab Road.  I do find it funny that I-74 goes to one lane at the Raab Road interchange.

[Revive 755]

Let's assume the capacity of an interstate lane is 2000 vehicles per hour (I'm finding in a traffic engineering book 2,350 vph for 65 mph, but I like a simpler number).  So both NB I-55 and WB I-74 can handle up to 4000 vph pre-merge.  So unless through NB I-55 and WB I-74 traffic totals at 4,000 vph, the multiplexed section will work fine with two lanes.  Unless the total through traffic is above 6,000 vph, three lanes will work.  The actual numbers will be less if there is heavy weaving involved post-merge.

I do find it funny that I-74 goes to one lane at the Raab Road interchange.

I don't; those single lane mainlines aren't fun with trucks that slow way down for the ramps.

[Chris]

Most traffic volumes on rural freeways in the United States are rather low, usually around 30,000 or less.  (there are some exceptions). If you combine that, you would get 60,000 vehicles per day, which is still acceptable on a 4 lane freeway. Above 70,000 AADT, I would consider 6-laning it.

[Alps]

If you combine that, you would get 60,000 vehicles per day, which is still acceptable on a 4 lane freeway. Above 70,000 AADT, I would consider 6-laning it.

At an average 8% (for urban areas) AADT during peak hour, and assuming capacity of 2,400 passenger cars per hour (about 2100 vehicles all told, with a decent truck percentage of 15% or so), you would end up with 30,000 per lane.  Capacity - which means you can't squeeze a single new car on the road (in theory) comes at 60,000 vehicles per day.  Really you would want to avoid capacity even during the peak hour, so head to the LOS D/E threshold (when flow is becoming turbulent) at 90% - so 54,000 vpd should be the threshold in rural areas.  In urban areas the peak period % rises toward 12%, so your vpd threshold would be even lower, more like 33,000 vpd.

[Duke87]

Another thing to keep in mind is that in reality capacity is not a matter of simple addition. Factors other than the number of vehicles and number of lanes contribute to how well traffic flows, such as visibility and width of the lanes.

Not to mention that it's not an entirely accurate assumption that the capacity of an 8 lane highway is equal to that of two equivalent four-lane highways.
Going from two lanes to four more than doubles the capacity, since the extra lane in each direction means people will be slowing down a lot less. On the other hand, as you keep increasing the number of lanes, it can actually start to do more harm than good... the inner lanes essentially start to become dead weight, since they're so far over from any exit or entrance ramp that people who aren't travelling long distance can't really make use of them... and of course sometimes people will find themselves in the left lane only to realize that their exit is coming up and have to make a mad dash to shift over... which of course creates problems both in terms of capacity and safety. The I-75/85 multiplex in Atlanta likely suffers from this. And it's also why it's a good thing that the northern sections of the New Jersey Turnpike are in a 2-3-3-2, 3-3-3-3, and 4-3-3-4 configuration, rather than just 5-5, 6-6 and 7-7. At more than four lanes in each direction it starts becoming beneficial to split traffic up if you can.

[Bryant5493]

... and of course sometimes people will find themselves in the left lane only to realize that their exit is coming up and have to make a mad dash to shift over... which of course creates problems both in terms of capacity and safety. The I-75/85 multiplex in Atlanta likely suffers from this.

Yeah, a lot of weaving goes on there -- it's unbelievable! It's nice and wide. Sometimes, this freeway is too wide for its own good.


Be well,

Bryant

[DrZoidberg]

I thought of another multiplex question that seems to fit this thread.

"Are Interstates 90 and 94 the only interstates that multiplex with each other more than once?"

(Madison, WI to Tomah, WI and again through downtown Chicago)

[Voyager]

In a lot of places it's not needed or pratical to do that. In other places it might be a good idea.

[Revive 755]

"Are Interstates 90 and 94 the only interstates that multiplex with each other more than once?"

I do not believe so.  There are very few interstates that even meet more than once if you exclude three digit routes and duplicate routes like the two I-76's:
* I-10 and I-17
* I-10 and I-12
* I-93 and I-95

Past occurences:
* I-94 and I-96 - per Kurumi's site, I-196 was once designated as I-96

Future occurrences:
* I-64 and I-70  - this one will probably only be for a limited time if the I-70 approach to the new Mississippi River bridge gets built (the one from I-55 east of IL 203, the first round of construction will have I-64 and I-70 bumping at the current tri-level interchange in Illinois)
* I-73 and I-74 - counting both routes are every fully completed
* I-73 and I-75 - counting I-73 ever makes it into Michigan
* I-5 and whatever number CA 99 ends up with

So I think I-90 and I-94 are the only routes that multiplex twice unless I-73 and I-74 ever get completed.

[DrZoidberg]

So I think I-90 and I-94 are the only routes that multiplex twice unless I-73 and I-74 ever get completed.

That's what I was getting at.  I believe 90/94 are the only interstates that multiplex together more than once, mostly because 94 is a north-south freeway through Illinois.

[Sykotyk]

I-94 is a mess, really. It touches I-90 near Billings (MT), Tomah (WI), Madison (WI), Chicago (IL) twice, Gary (IN) before finally departing.

To me, someone mentioned, I-94 should follow I-96 to Muskegon, have a gap across Lake Michigan, and start again in Milwaukee. I-94 from Gary (IN) to Detroit should be I-92. I-94 from Milwaukee to downtown Chicago should be I-57. I-94 from I-80 to I-57 should be a 3di (preferably a x57).

Or, if they don't want a gap, Just rename I-94 from Gary to Port Huron as I-92. And just omit I-94 from Michigan.

Sykotyk

[Duke87]

Another interesting question:

I-80 and I-90 merge for a good 270 miles. I seem to recall hearing somewhere that this is the longest multiplex between two highways in the world. Is that true?

[Coelacanth]

The multiplex of I-94 and US 52 between Jamestown and St. Paul is about 337 miles.

Of course over 200 of those miles are unsigned, but still.

[Darkchylde]

Future occurrences:
* I-64 and I-70  - this one will probably only be for a limited time if the I-70 approach to the new Mississippi River bridge gets built (the one from I-55 east of IL 203, the first round of construction will have I-64 and I-70 bumping at the current tri-level interchange in Illinois)
* I-73 and I-74 - counting both routes are every fully completed
* I-73 and I-75 - counting I-73 ever makes it into Michigan
* I-5 and whatever number CA 99 ends up with

Also, I-10 and I-49, when I-49's southern portion is completed (western junction currently exists in Lafayette, LA as I-49's south terminus - the eastern of these junctions is where I-10 currently junctions Business US 90/hidden I-910 in New Orleans.)

[leifvanderwall]

This is my opinion on I-94, I have known the interstate for 35 years: I-94 is the alternate route to I-90 and it was envisioned that way. I-90 gives you a more direct route to the east and west endpoints and I-94 goes to places I-90 can't for example Milwaukee, the Twin Cities and Detroit. Many people think it is ridiculous to have two interstates meet 4 times and multiplex twice, but the I-90/94 in Wisconsin & Chicago has the traffic volume of two highways anyways. I really think I-92 could be used in the New England area for example the Rt. 2 corridor in Massachusetts connecting Albany & Boston.

[Morriswa]

What is the most number of Interstate highways to be concurrent, even for a mile or less?

[Beltway]

I was, yes, thinking about interstate highway multiplexes, (specifically, looking at the I-55/I-74 multiplex in Bloomington, IL and Normal, IL) when I realized something.

Shouldn't multiplexed interstates carry the amount of lanes each multiplexed road carries going into the multiplexed combined?

Think about it. Traffic from two interstates combine, so even in rural areas, traffic going different places coming from different places combines. Wouldn't this theoretically mean double the traffic?

Depends on the place.  I can think of two that add only about 10% more traffic to the main route, I-95/I-64, and I-81/I-64, and neither of the main routes add lanes (I-95 has 6 lanes and I-81 has 4 lanes).

The overlap junctions allow traffic to go either way, for example a vehicle on EB I-64 at the Bryan Park Interchange can either join the SB I-95/EB I-64 overlap, or join NB I-95.  Plenty of traffic does the latter.  The same situation is true at the Downtown Interchange of I-95 and I-64.

The I-81/I-77 overlap does have a lot more traffic than the separate Interstates, but it also has more lanes (six) than the separate Interstates (4 each).

[drummer_evans_aki]

What is the most number of Interstate highways to be concurrent, even for a mile or less?

There are three. I-39/I-90/I-94 in Wisconsin and I-55/I-64/I-70 in Missouri/Illinois (short concurrency going over the Mississippi River

[national highway 1]

Another interesting question:

I-80 and I-90 merge for a good 270 miles. I seem to recall hearing somewhere that this is the longest multiplex between two highways in the world. Is that true?

The multiplex of I-94 and US 52 between Jamestown and St. Paul is about 337 miles.

Of course over 200 of those miles are unsigned, but still.

The multiplex of I-25 and US 87 is 609 miles between Raton, NM and Buffalo, WY.

[mcdonaat]

The longest highway multiplex I know of is I-20/I-59, at least the longest in my area. The only difference is that 20 and 59 meet at one end in Meridian, and the other in Birmingham, where both freeways expand to six lanes or more. If its only 1000 VPH for 59 and 1200 VPH for 20, and a 70 mph freeway can hold 3000, it's fine. I made those up, but you get my point. Sometimes traffic is so light that it wouldn't warrant widening. Example would be 49 where the speed limit is 75