There is a study of the Madison Beltline underway. http://www.dot.wi.gov/projects/swregion/madisonbeltline/index.htm. I live in Madison and have a great interest in the most heavily traveled route in my city. Has anyone else ever traveled along the Beltline, and if so, what improvements would you make?
It's been a few years since I was last there (for the record, I love that town, one of my favorites) and maybe I hit it at the right time, but I moved along it pretty quick.
Are there direct connectors from WB 12,14,18/SB 151 onto WB 18/SB 151? Looks like, from Google Earth, there is construction at this interchange.
And hey...how about I-139, so long as WisDot is putting up new Interstate shields statewide for 41. :D
I am a Madison native who can go back to the day when the big decision was to relocate or simply "improve" the east Beltline along Broadway. I can't imagine how bad it would be had they went with the "improve" decision back then!
Anyway, here are some thoughts:
**Three lanes from Midvale Boulevard west to Middleton. The two lanes are inadequate now.
**Eliminate the Todd Drive and Seminole Highway exits between Fish Hatchery and Midvale. Eliminate the Rimrock Road exit. There are simply too many entrance and exit points for an urban highway.
**Create an exit along I-39/90 to be used as alternatives to the Interstate to Beltline to Stoughton Road movement. Maybe extend Plaum Road east and a little north, which can then tie into Agriculture and put an interchange there? Or maybe a McFarland exit south of the Beltline at Sigglekow or Highway MN? I think a lot of the morning backups are traffic simply using that one small stretch of the Beltline to get onto Stoughton Road, and a lot of that could be fixed by keeping that traffic off the Beltline to begin with.
**Of course completing the North Beltline would be best.
I have lived in Madison for 10 years, grew up in Janesville so I know the area fairly well.
Three lanes of the West Beltline to Middleton would be great. The recent auxiliary lanes between Mineral Pt Rd and Old Sauk has been a great improvement.
I do want to see a North Beltline constructed, or at least a better route. I currently use US 12 to CTY K to CTY M to get over to WI 113. If it is constructed though, I don't really want to see any interchanges along the route. Not a fan of sprawl and a strong believer that people should live close to their work.
While I use the Seminole Hwy exit, mostly to avoid Verona Road, I wouldn't mind it being shuttered. That is the proposed plan once the 18/151 free-flow interchange is constructed in the distant future. What I would like to see is a C-D system from Todd Drive to John Nolen Drive (give or take depending if certain interchanges go away). This section is always highly congested as way too many people use the Beltline for local traffic. Anything that stops the amount of weaving traffic and too many interchanges would be appreciated for those using the road for regional traffic. If the C-D lanes get busy, I would be fine with that!
Regarding SEWIGuy and WarrenWallace's comments, here's some notes from the Origin-Destination study (and others related to the Beltway study):
- Less than 8% of Beltway traffic is through traffic that has origins and destinations outside of Dane County.
- "Very little traffic" drives the Beltway from end-to-end.
- Over half of traffic that enters the Beltway during the morning and evening peak exits within 4 interchanges.
- Almost one-fourth of westbound traffic on the Beltway between South Towne and John Nolen Dr is entering from Stoughton, Monona, or South Towne, and exiting at John Nolan, Rimrock, or Park St.
- On average, just under 8,000 vpd cross into Dane County on eastbound US 12 from Sauk City. Of this, only about 600 are traveling along routes where they would potentially use a North Beltline as an alternative. Only 330 of them continue east of Madison on US 12/18 or south on I-39/90.
- There is just under 4,000 vpd of through traffic on US 151 (i.e. entering AND departing Dane County along US 151).
- Northbound I-39/90 has approximately 22,600 vpd north of US 51/Exit 156. Of this, approximately 5,500 is through traffic continuing north of Madison on 39/90/94.
- Eastbound US 12 north of US 14 West in Middleton carries just over 2,800 vehicles during the AM peak hour. Only 1/6 of them are still on the Beltway at Verona Rd.
- Northbound Verona Rd at the Beltway carries about 2,900 vehicles during the AM peak hour. One-third of them continue north of the Beltway on Midvale Blvd. About half get on the eastbound Beltline. Of those taking the Beltline eastbound, almost half are exiting by John Nolan Dr. Another quarter are continuing to northbound 39/90.
- Of westbound Beltline traffic between 39/90 and Stoughton Rd during the AM peak hour (of which over half is coming from southbound 39/90), 20% is exiting onto northbound Stoughton Rd. 60% of it exits the Beltline by Verona Rd. The Stoughton Rd number confirms SEWIGuy's observation.
- The same location (westbound Beltline between 39/90 and Stoughton Rd) during the PM peak hour sees 80% of the traffic exit at or before Verona Rd.
- Almost half the traffic getting on the Beltline from John Nolen Dr during the PM peak hour goes eastbound and exits by Stoughton Rd.
- During the AM peak hour, almost 40% of southbound US 151 traffic near 39/90/94 heads to the Beltline. Another 40% stays on US 151 south of 39/90/94.
- The study has already concluded that the North Beltline would do little to help the South Beltline, and if the option south of Waunakee is chosen, it would actually increase traffic on the West Beltline (especially from Middleton north).
- A "South Reliever" corridor was looked at which would go more or less due east from the Verona bypass, cross 51 near Lake Kegonsa, and junction with 39/90 near CTH MN. While it would pull some traffic off the Beltline (up to 8-10K VPD from the stretch near Stoughton Rd, but only 1K west of Fish Hatchery Rd), it was concluded that it didn't do enough for the Beltway to work as a "stand-alone solution".
OK, thanks Froggie. So a few observations to take from this.
**The Beltline traffic is almost an entirely local and commuter issue, and Madison's geography (the lakes) hampers potential solutions that are alternatives to the Beltline.
**My idea for a new I-39/90 interchange between the I-94 interchange and the US-12/18 interchange might actually be feasible.
**Pretty much every solution has to be about increasing capacity, while also increasing safety. Adding lanes, closing exits, etc.
Quote from: SEWIGuy on April 17, 2015, 09:48:48 AM
**My idea for a new I-39/90 interchange between the I-94 interchange and the US-12/18 interchange might actually be feasible.
According to slide 9 in the I-39/90/94 study, there will be no added interchanges on the interstate between US 151 and the Beltline.
http://www.wisconsindot.gov/projects/swregion/399094/docs/ss20150113.pdf
A few thoughts:
- I have never thought of a North Beltline as being a reliever for the South Beltline, only as a reliever for the surface roads that it would 'bypass'. Yes, it will add traffic to US 12 to its northwest. Is that a reason not to build it? No. Just design everything with that in mind.
- What's the current potential/likelihood for a free-flowing interchange at Stoughton Rd?
- When the time comes for free-flow ramps at Verona Rd (they will be needed sooner than later, IMHO), I would try to include such ramps for the south <-> west turns in addition to the south <-> east turns. Build them over the Home Depot parking lot, if necessary. Those new I-41 interchange ramps in the Green Bay area are a good model to follow and promote for this location and possibly at Stoughton Rd.
- I also agree that consideration should be given to eliminating one or more of the street interchanges along the way and upgrading to six lanes to at least US 14 (west). (MAN! I can still remember Beltline/Old Sauk as a rural intersection with a two-lane surface highway!!! :wow: )
- Is the Beltline now the busiest non-interstate freeway in the USA's Midwest?
Mike
Here is some info about Stoughton Rd and in the area of the Beltline:
http://dot.state.wi.us/projects/swregion/51/altrec.htm
One tough thing with the Beltline, almost all roads that cross it has an interchange with it. I can only think of 3 roads right now that go over/under without an interchange: Terrance Ave in Middleton, High Point in west Madison, Agriculture Dr in far east Madison.
Quote from: WarrenWallace on April 17, 2015, 10:16:23 AM
Quote from: SEWIGuy on April 17, 2015, 09:48:48 AM
**My idea for a new I-39/90 interchange between the I-94 interchange and the US-12/18 interchange might actually be feasible.
According to slide 9 in the I-39/90/94 study, there will be no added interchanges on the interstate between US 151 and the Beltline.
http://www.wisconsindot.gov/projects/swregion/399094/docs/ss20150113.pdf
Yeah those are some legitimate concerns.
Quote from: WarrenWallace on April 17, 2015, 11:52:09 AM
One tough thing with the Beltline, almost all roads that cross it has an interchange with it. I can only think of 3 roads right now that go over/under without an interchange: Terrance Ave in Middleton, High Point in west Madison, Agriculture Dr in far east Madison.
That is incredible. I never have thought about it that way before.
The only two things that really need to be done on the Beltline are
1. Rebuild and Widen the Beltline from four to six lanes between Verona Road and US 14 in Middleton and eight lanes from Verona Road east to I-39/90 with wide shoulders for buses (not sure if this is in any plans)
2. Construct a free flow at the Beltline and U.S 18/151/Verona Road and eliminate the Seminole Highway interchange in the process.
3. Rebuild/Reconfigure the I-39/90/Beltline interchange
Yep.... I'm old enough to remember when the freeway ended after Mineral Point, as well as when there was an active railway crossing. I went to Madison frequently for school field trips, so I can remember many instances of being in that bus that stopped on the side of an urban freeway with traffic whirring by.
I haven't been on it in 15 years so I don't really know what it's like these days, but my thoughts have always been that it was underbuilt a little bit, even back then.
Is there enough urban sprawl on the north side of the lakes now to justify some kind of connection?
Quote from: mrose on April 17, 2015, 03:13:43 PM
Yep.... I'm old enough to remember when the freeway ended after Mineral Point, as well as when there was an active railway crossing. I went to Madison frequently for school field trips, so I can remember many instances of being in that bus that stopped on the side of an urban freeway with traffic whirring by.
There were two railroad crossings.
One just west of the Fish Hatchery Road exit (like you, I had to stop at that one in a school bus). There was also one just west of the Midvale Blvd / Verona Road exit.
Quote from: SEWIGuy on April 17, 2015, 03:23:46 PM
Quote from: mrose on April 17, 2015, 03:13:43 PM
Yep.... I'm old enough to remember when the freeway ended after Mineral Point, as well as when there was an active railway crossing. I went to Madison frequently for school field trips, so I can remember many instances of being in that bus that stopped on the side of an urban freeway with traffic whirring by.
There were two railroad crossings.
One just west of the Fish Hatchery Road exit (like you, I had to stop at that one in a school bus). There was also one just west of the Midvale Blvd / Verona Road exit.
I remember both of those grade crossings. (I also remember when Toys R Us used to be in the strip mall where Home Depot/Staples is now.) Now, both are home to bike path overpasses. If you look close enough at the satellite of the one near Verona Rd, you can barely pick out the RxR pavement markings.
The Beltline has suffered from decades of being cobbled together; a patchwork of spot improvements with no vision for the entire corridor. So this study is long, long overdue.
I think the middle portion of the Beltline (Verona Rd to South Towne) would benefit from Texas-style frontage roads in addition to the removal of some exits/entrances. That stretch already has frontage roads, so turning them into one-way roads and adding U-turns at all over/under crossings would be an economical way to improve the flow of everything with minimal takings.
The downside is, it would preclude the use of some alternative designs for service interchanges. For example, Fish Hatchery Rd is a good candidate for a SPUI or a DDI, but those won't work with TX-style frontage roads.
The Beltline interchange with 39/90; the alternative WisDOT is leaning towards is a full turbine design.
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi113.photobucket.com%2Falbums%2Fn208%2Ftriplemultiplex%2FInterchanges%2Fbeltline%2520snip_zpsfmm4xglh.jpg&hash=f4b5a757a80fd7b7b8b54dc2c677469ed32aa229)
Something that is reasonable but for one ramp. WB->SB does not need to be that elaborate. Certainly not worth the millions of extra dollars it would cost to build the high-speed version of this ramp. The loop ramp will be more than sufficient as this is the lowest-volume movement.
Quote from: froggie on April 16, 2015, 06:42:30 PM
Regarding SEWIGuy and WarrenWallace's comments, here's some notes from the Origin-Destination study (and others related to the Beltway study):
- On average, just under 8,000 vpd cross into Dane County on eastbound US 12 from Sauk City. Of this, only about 600 are traveling along routes where they would potentially use a North Beltline as an alternative. Only 330 of them continue east of Madison on US 12/18 or south on I-39/90.
Someone in Baraboo or the Dells destined for the east side of Madison would more likely use I-39/90/94 instead, or cut across via Hwy 60. Trying to cut across Waunakee via Hwy 19 during the morning commute is just a bad idea. If a northside leg to the Beltline was constructed, however, you'd see more traffic using US-12.
Quote from: froggie on April 16, 2015, 06:42:30 PM
- There is just under 4,000 vpd of through traffic on US 151 (i.e. entering AND departing Dane County along US 151).
- Northbound I-39/90 has approximately 22,600 vpd north of US 51/Exit 156. Of this, approximately 5,500 is through traffic continuing north of Madison on 39/90/94.
Making the reverse commute (Madison -> Janesville) for a few years, I can attest to the significant traffic on NBD I-39/90 during the morning commute, and SBD during the evening commute.
Quote from: froggie on April 16, 2015, 06:42:30 PM
- Eastbound US 12 north of US 14 West in Middleton carries just over 2,800 vehicles during the AM peak hour. Only 1/6 of them are still on the Beltway at Verona Rd.
Makes complete sense, given my earlier commentary.
Quote from: triplemultiplex on April 19, 2015, 12:28:21 PM
I think the middle portion of the Beltline (Verona Rd to South Towne) would benefit from Texas-style frontage roads in addition to the removal of some exits/entrances. That stretch already has frontage roads, so turning them into one-way roads and adding U-turns at all over/under crossings would be an economical way to improve the flow of everything with minimal takings.
See I just don't think you need any frontage row exits. Remove Rimrock, because you can just use John Nolen, and remove Seminole and Tood Drive because you can just use Midvale or Fish Hatchery.
Quote from: SEWIGuy on April 19, 2015, 03:07:13 PM
See I just don't think you need any frontage row exits. Remove Rimrock, because you can just use John Nolen, and remove Seminole and Tood Drive because you can just use Midvale or Fish Hatchery.
Ditto. While they may be semi-convenient, they're just leftovers from days gone by on the beltline. Spend that money improving the surface streets in the area.
Quote from: triplemultiplex on April 19, 2015, 12:28:21 PM
The Beltline has suffered from decades of being cobbled together; a patchwork of spot improvements with no vision for the entire corridor. So this study is long, long overdue.
I think the middle portion of the Beltline (Verona Rd to South Towne) would benefit from Texas-style frontage roads in addition to the removal of some exits/entrances. That stretch already has frontage roads, so turning them into one-way roads and adding U-turns at all over/under crossings would be an economical way to improve the flow of everything with minimal takings.
The downside is, it would preclude the use of some alternative designs for service interchanges. For example, Fish Hatchery Rd is a good candidate for a SPUI or a DDI, but those won't work with TX-style frontage roads.
The Beltline interchange with 39/90; the alternative WisDOT is leaning towards is a full turbine design.
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi113.photobucket.com%2Falbums%2Fn208%2Ftriplemultiplex%2FInterchanges%2Fbeltline%2520snip_zpsfmm4xglh.jpg&hash=f4b5a757a80fd7b7b8b54dc2c677469ed32aa229)
Something that is reasonable but for one ramp. WB->SB does not need to be that elaborate. Certainly not worth the millions of extra dollars it would cost to build the high-speed version of this ramp. The loop ramp will be more than sufficient as this is the lowest-volume movement.
I do think though that the EB Beltline to NB 39/90 ramp should be 2 lanes and able to be taken at 55 mph. There's enough commuter traffic and if 151 in the area is ever given Interstate status to warrant that. Wisconsin is usually pretty good at building for with a big picture future in mind.
These are all very interesting ideas. I would add a lane in each direction from Verona Road to the Interstate and congestion-price it. I would widen all shoulders and lanes to the standard 12 feet width. I would eliminate the Seminole Hwy interchange and utilize the ramps for the Verona Rd. interchange. The lack of additional over/underpasses without interchanges does pose problems. More will probably be added to the road in the future.
QuoteI do think though that the EB Beltline to NB 39/90 ramp should be 2 lanes and able to be taken at 55 mph.
2 lanes, yes (as with the SB 39/90 to WB Beltline ramp). 55 MPH isn't necessary, especially if it would have significant constructability or right-of-way impact.
Quote from: The Ghostbuster on April 20, 2015, 05:15:56 PM
These are all very interesting ideas. I would add a lane in each direction from Verona Road to the Interstate and congestion-price it. I would widen all shoulders and lanes to the standard 12 feet width. I would eliminate the Seminole Hwy interchange and utilize the ramps for the Verona Rd. interchange. The lack of additional over/underpasses without interchanges does pose problems. More will probably be added to the road in the future.
Safe to say, congestion pricing isn't going to be on the table for the Beltline.
Quote from: froggie on April 20, 2015, 10:37:47 PM
QuoteI do think though that the EB Beltline to NB 39/90 ramp should be 2 lanes and able to be taken at 55 mph.
2 lanes, yes (as with the SB 39/90 to WB Beltline ramp). 55 MPH isn't necessary, especially if it would have significant constructability or right-of-way impact.
No, 55 mph or better is necessary for a freeway to freeway ramp. Speed should be able to be maintained between freeways.
Quote from: Brandon on April 20, 2015, 11:14:32 PM
Quote from: froggie on April 20, 2015, 10:37:47 PM
QuoteI do think though that the EB Beltline to NB 39/90 ramp should be 2 lanes and able to be taken at 55 mph.
2 lanes, yes (as with the SB 39/90 to WB Beltline ramp). 55 MPH isn't necessary, especially if it would have significant constructability or right-of-way impact.
No, 55 mph or better is necessary for a freeway to freeway ramp. Speed should be able to be maintained between freeways.
Agreed. In fact there probably is enough ROW in the current interchange footprint that it could be done without needing much more ROW. Of course if people didn't have a phobia of left exits and entrances, it could easily be done. :whip:
Phobia?
You mean "accept the consensus opinion that left-hand movements (at asymmetrical merges/diverges) impede the smooth operation of freeway traffic"? :p
Since this is not available anymore, here's a snip of a WisDOT proposal for the I-39/90 - Beltline interchange in the early days of the interstate expansion that's starting to ramp up now.
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi113.photobucket.com%2Falbums%2Fn208%2Ftriplemultiplex%2FInterchanges%2Fbeltline%2520oldsnip_zpsvtrmkyua.jpg&hash=62906542e0dacbd3461ed38f791444c746d69faa)
I would describe this as an order of magnitude cheaper then the last one of WisDOT's I posted. And just as effective for moving traffic through this interchange better than the existing setup.
Yes I call it a phobia of some, both here and in various DOTs, that have a need to replace every single left exit and entrance. At this interchange, there is plenty of distance between US 51 and I-39/90 as well as between US 12 and I-94/Wis 30 that left hand exits could remain. But that's just me.
QuoteNo, 55 mph or better is necessary for a freeway to freeway ramp. Speed should be able to be maintained between freeways.
For those who have a phobia for using a brake pedal, perhaps. But it's not always feasible for any of a multitude of reasons. Slowing down to 45 MPH for a freeway ramp isn't the end of the world, you know...
That said, I haven't looked at the specific geometry for 39/90 at the Beltline (yet), so what hobsini posted might be possible.
I still think some sort of lane pricing should be implemented, otherwise the road will just fill back up. Not that I'm a proponent of the "induced travel" theory.
Quote from: froggie on April 22, 2015, 09:53:08 AM
QuoteNo, 55 mph or better is necessary for a freeway to freeway ramp. Speed should be able to be maintained between freeways.
For those who have a phobia for using a brake pedal, perhaps. But it's not always feasible for any of a multitude of reasons. Slowing down to 45 MPH for a freeway ramp isn't the end of the world, you know...
That said, I haven't looked at the specific geometry for 39/90 at the Beltline (yet), so what hobsini posted might be possible.
Froggie, having grown up in Central Wisconsin and Northern Illinois, I can tell you that geometrically 39/90 at the Beltline is one of the few places in this part of the Midwest you can have a 55 mph 2 lane ramp in the current footprint from end to end.
The current EB-NB ramp can be taken at 55 in a car quite easily.
Quote from: triplemultiplex on April 25, 2015, 11:16:10 AM
The current EB-NB ramp can be taken at 55 in a car quite easily.
Or at 60 with dry conditions.
I've done it at 65-70, but anyone riding with me who's paranoid about tipping vehicles would have a shit-fit :bigass:
Quote from: SSOWorld on April 25, 2015, 01:56:19 PM
I've done it at 65-70, but anyone riding with me who's paranoid about tipping vehicles would have a shit-fit :bigass:
I've done that as well, but only if no one else is in the car. My fiancée grabs the oh-shit handle if I take it anywhere near 60. The car on the other hand, takes it like a champ.
Quote from: DaBigE on April 25, 2015, 03:04:35 PM
Quote from: SSOWorld on April 25, 2015, 01:56:19 PM
I've done it at 65-70, but anyone riding with me who's paranoid about tipping vehicles would have a shit-fit :bigass:
I've done that as well, but only if no one else is in the car. My fiancée grabs the oh-shit handle if I take it anywhere near 60. The car on the other hand, takes it like a champ.
Exactly :happy:
Pardon the bump, but here's a question I've had about the Beltline: has there been any talk about bringing it east of I-39/90 and having it swing north to meet I-94 east of the city? Presumably this "East Beltline Extension" would have an exit with Highway N south of Cottage Grove, as well as Highway BB to the east of that town, but where would it junction with I-94? I'm guessing either it would tie into the existing Highway 73 exit (near Deerfield) in some fashion, or a new interchange would be built a little to the west or east of it.
Anyway, this is just speculation on my part, and as far as I know, there are no official proposals for such a road from WisDOT. What are your thoughts?
Quote from: KCRoadFan on December 15, 2021, 01:42:54 AM
Pardon the bump, but here's a question I've had about the Beltline: has there been any talk about bringing it east of I-39/90 and having it swing north to meet I-94 east of the city? Presumably this "East Beltline Extension" would have an exit with Highway N south of Cottage Grove, as well as Highway BB to the east of that town, but where would it junction with I-94? I'm guessing either it would tie into the existing Highway 73 exit (near Deerfield) in some fashion, or a new interchange would be built a little to the west or east of it.
Anyway, this is just speculation on my part, and as far as I know, there are no official proposals for such a road from WisDOT. What are your thoughts?
My thoughts are that it would be a duplicate of I-39/90 and just not necessary. Expansion of I-39/90 would just be easier and less expensive.
Also, east of the interstate, US-12/18 is primarily for local and regional traffic. Expanding it as necessary as the suburbs grow east is likely sufficient.
There has not been any proposal, real or fictional for that matter, of this particular Beltline eastern extension to I-94. I can't imagine such a facility would take very much traffic off of 39/90 since most of the Beltline traffic coming and going north is bound for the East Town area around US 151. Lots of traffic heads to/from Sun Prairie or any of the sprawling big box stores in that general area, not to mention AmFam's HQ.
It's actually amazing how little traffic on 39/90 NB gets off on 94 EB compared to how much keeps chugging north to the next exit. I think that's a result of how moderate the sprawl due east is compared to northeast. And the reason is simple: interchange density. There's a bunch of exits on US 151 as it goes north from the interstate while there is only one on 94. This makes it harder to get to the freeway and discourages the type of sprawl in Cottage Grove that one sees in Sun Prairie.
Oh don't get me wrong, there's sprawl in Cottage Grove, but if there were two more exits between CTH N and 39/90, 94 would look like 151 with sprawl everywhere and daily slowdowns from commuters rather than fast-moving with mostly farms flanking the freeway.
Quote from: triplemultiplex on December 15, 2021, 05:07:30 PM
It's actually amazing how little traffic on 39/90 NB gets off on 94 EB compared to how much keeps chugging north to the next exit.
I'm guessing that's because the primary beneficiary of that particular traffic movement (people heading from the Beloit/Janesville area to Milwaukee) would just be taking I-43 instead - right?
Quote from: KCRoadFan on December 15, 2021, 05:58:46 PM
Quote from: triplemultiplex on December 15, 2021, 05:07:30 PM
It's actually amazing how little traffic on 39/90 NB gets off on 94 EB compared to how much keeps chugging north to the next exit.
I'm guessing that's because the primary beneficiary of that particular traffic movement (people heading from the Beloit/Janesville area to Milwaukee) would just be taking I-43 instead - right?
Depending on what part of Milwaukee I'm headed, I have a choice of either Hwy 26 or I-43. There's no reason for me to drive all the way to Madison first.
Quote from: KCRoadFan on December 15, 2021, 05:58:46 PM
Quote from: triplemultiplex on December 15, 2021, 05:07:30 PM
It's actually amazing how little traffic on 39/90 NB gets off on 94 EB compared to how much keeps chugging north to the next exit.
I'm guessing that's because the primary beneficiary of that particular traffic movement (people heading from the Beloit/Janesville area to Milwaukee) would just be taking I-43 instead - right?
No. It's traffic coming off the Beltline going home. There's very little suburban growth going east on I-94 to Milwaukee. Cottage Grove has gotten bigger in the last forty years, but it's nothing compared to the growth Sun Prairie has experienced. The best way from Sun Prairie to the jobs on the west side (Epic, Research Park) is via the Beltline.
It's also the best way to get to the Fox Valley for Janesville, Beloit and if you are coming into Wisconsin from the south, but that's not the main reason for the difference in traffic levels.