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[MCRoads]

I think that both over-freeway and under-freeway SPUIs have trade-offs: a wider bridge with short span lengths, and narrower bridge with longer spans, respectively. Of corse, there are exceptions, such as those shown above, but I can also contribute:

I-29 @ E Front St, Kansas City, MO

MD-5 @ Allentown Rd, Camp Springs, MD (does this even count as a SPUI?)

Of corse, SPUIs don’t need to be over the freeway. Offset SPUIs are a thing, like I-225 @ E Alameda Ave, Denver, CO.

Or, there is the (often shit on) inverted SPUI, as seen on I-244, Tulsa, OK.

But I think my favorite SPUI I’ve seen isn’t even real! It was built in Minecraft! It is skewed enough that having straddle bents is feasible. I believe that there may need to be a small adjustment to the center piers so that small low-speed attenuators (similar to this) could be used, but I think it looks like something that actually might be built.

[webny99]

SPUI's are non-existent in this area, which is probably why I'm not used to seeing them under the freeway. It seems like a disaster waiting to happen for first-time users.

Then the accident rate would be thru the roof every time a new driver encounters one.

A SPUI is nothing more than a common left turn. At a typical intersection, opposing directions make simultaneous left turns all day long.

The left turns are much wider than typical left turns, and with that movement also happening under an overpass, I guarantee there would be plenty of confused drivers until they got used to it.  Of course, this is mostly conjecture for us in the Northeast because there are so few examples.

[MCRoads]

SPUI's are non-existent in this area, which is probably why I'm not used to seeing them under the freeway. It seems like a disaster waiting to happen for first-time users.

Then the accident rate would be thru the roof every time a new driver encounters one.

A SPUI is nothing more than a common left turn. At a typical intersection, opposing directions make simultaneous left turns all day long.

The left turns are much wider than typical left turns, and with that movement also happening under an overpass, I guarantee there would be plenty of confused drivers until they got used to it.  Of course, this is mostly conjecture for us in the Northeast because there are so few examples.

I think  in terms of “disasters wating to happen”, New England has more than a few. And none are SPUIs.

COUGH rotary intersections COUGH

[SkyPesos]

COUGH rotary intersections COUGH

I thought roundabouts are generally a good thing.

[MCRoads]

COUGH rotary intersections COUGH

I thought roundabouts are generally a good thing.

There is a difference between a good roundabout and a terrible rotary. Traffic usually has to change lanes in the circle to make left turns, and may have to yield to traffic entering the circle, or pedestrians crossing the circulating roadway. DC and Philly have the most I can think of off the top of my head.

[froggie]

MD-5 @ Allentown Rd, Camp Springs, MD (does this even count as a SPUI?)

Why wouldn't it?

(as a former regular user of that interchange, I don't see why it wouldn't count)

COUGH rotary intersections COUGH

I thought roundabouts are generally a good thing.

Besides what MCRoads posted, there are technical differences between the two.  Roundabouts have tighter radii and ALWAYS require entering traffic to yield to traffic already in the circle.

Rotaries have more in common with the old traffic circles, including the ones in DC and Philly that MCRoads mentioned.  One thing he may not know about though, is that in Massachusetts (where the term "rotary" came from), state law requires entering traffic to yield at rotaries, just like it does for roundabouts.  It's New Jersey in particular where you'll see the phenomenon of entering traffic having the right-of-way (Flemington Circle on US 202 coming to mind).

[MCRoads]

MD-5 @ Allentown Rd, Camp Springs, MD (does this even count as a SPUI?)

Why wouldn't it?

(as a former regular user of that interchange, I don't see why it wouldn't count)

COUGH rotary intersections COUGH

I thought roundabouts are generally a good thing.

Besides what MCRoads posted, there are technical differences between the two.  Roundabouts have tighter radii and ALWAYS require entering traffic to yield to traffic already in the circle.

Rotaries have more in common with the old traffic circles, including the ones in DC and Philly that MCRoads mentioned.  One thing he may not know about though, is that in Massachusetts (where the term "rotary" came from), state law requires entering traffic to yield at rotaries, just like it does for roundabouts.  It's New Jersey in particular where you'll see the phenomenon of entering traffic having the right-of-way (Flemington Circle on US 202 coming to mind).

FYI, even MA is not immune to the “New Jersey phenomenon“ , the Sullivan Square rotary is an example of users on the circulating roadway having to stop (via a signal on the SW quadrant), as well as the Neponset Circle (multiple signals/pedestrian crossings), and Harvord Square (the NE quadrant looks like it was probably a delightful mess to drive before the new-ish looking canalization islands were added, lol).

[1]

Massachusetts has a few where entering traffic from certain directions has priority: https://www.google.com/maps/@42.4551208,-71.0900419,18.75z

[TEG24601]

Massachusetts has a few where entering traffic from certain directions has priority: https://www.google.com/maps/@42.4551208,-71.0900419,18.75z

Seems normal to me, just a compacted Michigan Left.

[vdeane]

... partly because, again, SPUI's are literally non-existent in NY/NJ/PA, so I'm kind of flying blind here.

I commute on a SPUI every day.*

That's one of how many in the entire state... two? three?

In addition to what I noted for NY above, at least two.

We have at least five.  I-87/NY 7/NY 2, I-278/NY 25A, Nicolls Road/NY 25, I-781/US 11, and NY 5/8/12/Court Street (3/4 interchange) are the ones I can think of off the top of my head.  Of those listed, all except I-87 exit 6 have the freeway on top.

[froggie]

Massachusetts has a few where entering traffic from certain directions has priority: https://www.google.com/maps/@42.4551208,-71.0900419,18.75z

That's not, by either definition or engineering, a rotary.  That's a pair of U-turn ramps on either side of a RIRO.

[ErmineNotyours]

They have gone to the extreme in this map and simply eliminated all movements between the freeways and West Valley Hwy, apart from a singular westbound off-ramp.

There's also an east on-ramp, with access to north 167.

[SkyPesos]

For Ohio's SPUIs, like what has been noted for NY above, it seems like somewhat of a "foreign" concept here. There's almost 4x more in my former hometown of St Louis than all of Ohio, and none of them are under the freeway. A bit disappointing as it's my favorite freeway to surface interchange type. The state uses a lot of hybrid diamond interchanges with a loop ramp or two though.
Here's all the SPUIs in Ohio I can think of:
- I-75 and OH 63 (over)
- I-475 and US 20 (over)
- I-270 and Sawmill (over)
- OH 161 and Sunbury (over)

As for the St Louis examples...
- I-55 and US 50/61/67 (over)
- I-64 and US 61/67 (under)
- I-64 and Hampton (over)
- I-64 and Kingshighway (over)
- I-70 and TR Hughes (over)
- I-70 and MO 94 (over)
- I-70 and Florissant (under)
- I-170 and MO 340 (under)
- I-170 and MO 180 (under)
- I-270 and MO 340 (over)
- MO 141 and Big Bend (over)
- MO 141 and MO 100 (under)
- MO 141 and Ladue (over)
- MO 141 and MO 340 (under)
- MO 364 and MO K (under)
I like how Missouri does some of the signaling in their under interchanges, with horizontal signals under the overpass, instead of a vertical signal mounted above an "island" in the middle of the interchange I see sometimes.

[webny99]

... partly because, again, SPUI's are literally non-existent in NY/NJ/PA, so I'm kind of flying blind here.

I commute on a SPUI every day.*

That's one of how many in the entire state... two? three?

In addition to what I noted for NY above, at least two.

We have at least five.  I-87/NY 7/NY 2, I-278/NY 25A, Nicolls Road/NY 25, I-781/US 11, and NY 5/8/12/Court Street (3/4 interchange) are the ones I can think of off the top of my head.  Of those listed, all except I-87 exit 6 have the freeway on top.

However, none west of I-81 in either NY or PA that I'm aware of.. unless there are any in the Pittsburgh area that I missed.

[froggie]

^ BYPASS US 19/PA 51 at the south end of the Liberty Tunnel in Pittsburgh functions as a SPUI.  It's not a traditional SPUI, but functions as one.  I believe they went with this design (including the loop ramp) because the close proximity to the Liberty Tunnel prevented left turn lanes on the southbound approach.

[SkyPesos]

Speaking of interchanges that aren't SPUIs, but have parts functioning like one, here's I-55 and MO 141. There's 3 signal sets: the 2 side ones for the left turns on the freeway, and the middle one is for left turns off the freeway, and both directions of left turns can go at the same time. Even the signal head that was chosen for the left turn (diagonal left) is what MoDOT normally uses on SPUIs and DDIs.

[Steve.S]

For Ohio's SPUIs, like what has been noted for NY above, it seems like somewhat of a "foreign" concept here. There's almost 4x more in my former hometown of St Louis than all of Ohio, and none of them are under the freeway. A bit disappointing as it's my favorite freeway to surface interchange type. The state uses a lot of hybrid diamond interchanges with a loop ramp or two though.
Here's all the SPUIs in Ohio I can think of:
- I-75 and OH 63 (over)
- I-475 and US 20 (over)
- I-270 and Sawmill (over)
- OH 161 and Sunbury (over)

There's also I-71 and OH-665 in Grove City (over): https://www.google.com/maps/@39.8403284,-83.0907414,318m/data=!3m1!1e3.

[SkyPesos]

For Ohio's SPUIs, like what has been noted for NY above, it seems like somewhat of a "foreign" concept here. There's almost 4x more in my former hometown of St Louis than all of Ohio, and none of them are under the freeway. A bit disappointing as it's my favorite freeway to surface interchange type. The state uses a lot of hybrid diamond interchanges with a loop ramp or two though.
Here's all the SPUIs in Ohio I can think of:
- I-75 and OH 63 (over)
- I-475 and US 20 (over)
- I-270 and Sawmill (over)
- OH 161 and Sunbury (over)

There's also I-71 and OH-665 in Grove City (over): https://www.google.com/maps/@39.8403284,-83.0907414,318m/data=!3m1!1e3.

Thought I was missing one either in Cincy or Columbus. Thx.

[SkyPesos]

Here's one that was recently rebuilt: I-71's southern interchange with I-270. I'm pretty concerned about the 2 lanes each way for I-71 through traffic on both sides, with seemingly no easy way to widen this in the future. I think OhioDOT might have shot themselves in the foot by not widen it to 3 lanes when they had the chance to when reconstructing the whole thing, and it might eventually be a bigger issue when I-71 between Cincinnati and Columbus is fully 6-laned.

[ran4sh]

They probably expect the 2 thru lanes to be sufficient especially if Cincinnati traffic simply uses I-270 to bypass Columbus.

[SkyPesos]

They probably expect the 2 thru lanes to be sufficient especially if Cincinnati traffic simply uses I-270 to bypass Columbus.

Using I-270 to bypass Columbus isn’t as simple as you think. Adds an extra 8 miles compared to staying on I-71, and goes through the area (in my experience) with the worst traffic in the city; the segment between US 33/161 and I-71 on the north loop of I-270.