The windmill interchange offers nearly identical traffic flow to the stack

[zzomtceo]

This interchange, known as a windmill interchange is effectively almost the same as a stack. All traffic exits first on the right, then splits depending on direction, returning to the freeway on the right. The only difference is that lefthand turns join the freeway directly instead of joining the opposite direction's righthand turn ramp first. This difference  doesn't affect the flow of the traffic at all or create other problems, and is much cheaper.

[hotdogPi]

Is this something that exists, or is it something that you made up?

[1995hoo]

The curves appear to be a lot sharper, thus requiring traffic to slow down significantly more. That could be a serious disadvantage depending on the particular situation.

[hotdogPi]

Would left turns being entirely inside the center square work instead of them being long ramps? (Maybe if the lines were further apart)?

[zzomtceo]

The curves appear to be a lot sharper, thus requiring traffic to slow down significantly more. That could be a serious disadvantage depending on the particular situation.

The curves would vary based on implementation, they could be stretched and changed numerous ways, although probably not quite as good as stack.

[zzomtceo]

Would left turns being entirely inside the center square work instead of them being long ramps? (Maybe if the lines were further apart)?

Yes, but with the problem of left hand exits and merges

[english si]

Like this? OK, the ramps cross the road you've just left, rather than have a sharp curve.

It's called a Turban (or Turbine) Interchange, and isn't quite as capacious as a stack, though is cheaper to build.

[zzomtceo]

Like this? OK, the ramps cross the road you've just left, rather than have a sharp curve.

It's called a Turban (or Turbine) Interchange, and isn't quite as capacious as a stack, though is cheaper to build.

The turbine is very similar to the windmill, except that the last part of the ramp in the windmill left turn can be on the same bridge as the freeway it's going onto, which is possible on a turbine but would invoke the same curve issues as a windmill. That being said, a turbine is probably the best compromise, avoiding ramp stacking over 2 levels (possible, not always) while also avoiding sharp curves (again, can be avoided, but not always)

[vdeane]

For people who don't mind left exits/entrances, there's this thing: http://goo.gl/maps/8NtCz

NYSDOT currently plans to replace it with something that looks just like it but isn't, to eliminate the weave on NY 390 north.

[vtk]

I thought a windmill interchange was four diamond "exit" ramps, each starting with a gradual diverge and ending at a t intersection

[briantroutman]

I thought a windmill interchange was four diamond "exit" ramps, each starting with a gradual diverge and ending at a t intersection

Yes–according to Kurumi, it is. (http://www.kurumi.com/roads/interchanges/oddities.html) But I think that's just his informal name for it.

[kurumi]

I thought a windmill interchange was four diamond "exit" ramps, each starting with a gradual diverge and ending at a t intersection

Yes–according to Kurumi, it is. (http://www.kurumi.com/roads/interchanges/oddities.html) But I think that's just his informal name for it.

Yup, it's informal. (so is the Volleyball Interchange.)

The 490/390 interchange reminds me of a cat's cradle, and was planned for I-91/CT 9 (you can see where, when the more modern northern leg of CT 9 was constructed, the state revised two of the left exits into right-hand exits), and I-291/CT 9

[zzomtceo]

I thought a windmill interchange was four diamond "exit" ramps, each starting with a gradual diverge and ending at a t intersection

Yes–according to Kurumi, it is. (http://www.kurumi.com/roads/interchanges/oddities.html) But I think that's just his informal name for it.

I saw some website referring to the one I posted as a windmill, but terminology isn't very consistent around seldom used interchanges.
It occurs to me that the name "windmill" might be because of its similarity to the turbine interchange.

[tradephoric]

This is an interesting design.  Essentially it is a Parclo B4 interchange with the loop ramps in the median of the arterial street. 


It's a free-flowing interchange design that doesn't require flyover ramps (also, all traffic exiting and entering onto the freeway does so from the right hand side).  Wide arterial streets can also come in handy at major intersections to keep the flow of traffic moving.  Below is an example of a town center intersection.  Wide arterial medians allow for so much more flexibility with intersection/interchange designs (in the case of interchanges, free flowing interchanges are possible without the use of flyovers; in the case of intersections, simple 2-phase signal designs are possible without the need for wasteful left-turn phases).

[zzomtceo]

This is an interesting design.  Essentially it is a Parclo B4 interchange with the loop ramps in the median of the arterial street. 


It's a free-flowing interchange design that doesn't require flyover ramps (also, all traffic exiting and entering onto the freeway does so from the right hand side).  Wide arterial streets can also come in handy at major intersections to keep the flow of traffic moving.  Below is an example of a town center intersection.  Wide arterial medians allow for so much more flexibility with intersection/interchange designs (in the case of interchanges, free flowing interchanges are possible without the use of flyovers; in the case of intersections, simple 2-phase signal designs are possible without the need for wasteful left-turn phases).

If you closely examine the curved loop sections, they employ left hand turns for exits from the right and left freeway, in addition to being quite curved, moreso than the windmill or turbine, potentially slowing traffic.

[tradephoric]

^^^Design meant for freeway-to-arterial interchange (wouldn't work well for freeway-to-freeway interchanges since left-turn exits/entrances do exist along one of the roadways). 

[zzomtceo]

^^^Design meant for freeway-to-arterial interchange (wouldn't work well for freeway-to-freeway interchanges since left-turn exits/entrances do exist along one of the roadways).

While I agree with you, in the picture it appears (according to the coloring scheme of Google Maps) that the wrong road is a freeway, ideally it would be the north south road, which avoids lefthand exits.

[tradephoric]

^^^ It appears that the speed limit along Caroline Bays Parkway is reduced to 50 MPH in both directions before you get to the interchange... possibly for the left exit/entrance issue.  A similar design existed at Telegraph Road and I-94 before MDOT converted it to a SPUI (I-94 had left exits/entrances as well, which was less than ideal).

[hbelkins]

The loop ramps from the US route to the state route merge into the left lane.

[Brandon]

This is an interesting design.  Essentially it is a Parclo B4 interchange with the loop ramps in the median of the arterial street. 

Reminds me a lot of the former I-94/Telegraph Road (US-24) interchange.

Of course, with left ramps, you have the interchange at the Ford and the Lodge.

[Brandon]

This interchange, known as a windmill interchange is effectively almost the same as a stack. All traffic exits first on the right, then splits depending on direction, returning to the freeway on the right. The only difference is that lefthand turns join the freeway directly instead of joining the opposite direction's righthand turn ramp first. This difference  doesn't affect the flow of the traffic at all or create other problems, and is much cheaper.

It also looks a lot like this: Fisher Freeway & Lodge Freeeway.  Some of the ramps; however, come together prior to merging with the mainline.

[tradephoric]

This interchange, known as a windmill interchange is effectively almost the same as a stack. All traffic exits first on the right, then splits depending on direction, returning to the freeway on the right. The only difference is that lefthand turns join the freeway directly instead of joining the opposite direction's righthand turn ramp first. This difference  doesn't affect the flow of the traffic at all or create other problems, and is much cheaper.

Would traffic flow be improved with a windmill interchange since each ramp merges onto the freeway directly?  This leads to multiple merge points spaced roughly 700-100 feet apart.  This could do a better job at dispersing traffic and keeping it moving.  With a stack, you get the traffic volumes of multiple ramps merging at ONE merge point, seemingly leading to a bottleneck.  Just a thought. 

There is a tight stack interchange at I-75 & I-696 in Detroit.  It's quite common to get backed up (even during seemingly off-peak times) when traveling EB I-696 to go NB I-75.  I wonder if instead of ONE merge point, traffic flow would be improved if there were TWO merge points spaced roughly 1000 feet apart. 

I-75 & I-696 Stack interchange:
https://maps.google.com/?ll=42.477609,-83.112897&spn=0.004532,0.004823&t=h&z=18

[zzomtceo]

This interchange, known as a windmill interchange is effectively almost the same as a stack. All traffic exits first on the right, then splits depending on direction, returning to the freeway on the right. The only difference is that lefthand turns join the freeway directly instead of joining the opposite direction's righthand turn ramp first. This difference  doesn't affect the flow of the traffic at all or create other problems, and is much cheaper.

Would traffic flow be improved with a windmill interchange since each ramp merges onto the freeway directly?  This leads to multiple merge points spaced roughly 700-100 feet apart.  This could do a better job at dispersing traffic and keeping it moving.  With a stack, you get the traffic volumes of multiple ramps merging at ONE merge point, seemingly leading to a bottleneck.  Just a thought. 

There is a tight stack interchange at I-75 & I-696 in Detroit.  It's quite common to get backed up (even during seemingly off-peak times) when traveling EB I-696 to go NB I-75.  I wonder if instead of ONE merge point, traffic flow would be improved if there were TWO merge points spaced roughly 1000 feet apart. 

I-75 & I-696 Stack interchange:
https://maps.google.com/?ll=42.477609,-83.112897&spn=0.004532,0.004823&t=h&z=18

I think one of the best ways to reduce impact of merges is to just add new lanes and leave them there for a while rather than trying to quickly merge into the existing lanes, especially since lanes can be taken away (going to ramps) prior to the interchange. I do think you have a good point though and I've never considered it before. and the maps link did look like an extremely tight interchange that probably needs more space no matter what kind of interchange is being used, especially with a windmill which would need severely curved ramps in such a small space.
Also in a somewhat related note it's cool how many mentions of the Detroit area system there are in this post, I live by Lansing but I love the system in Detroit so much more, despite it also having outdated designs in many areas.

[zzomtceo]

This interchange, known as a windmill interchange is effectively almost the same as a stack. All traffic exits first on the right, then splits depending on direction, returning to the freeway on the right. The only difference is that lefthand turns join the freeway directly instead of joining the opposite direction's righthand turn ramp first. This difference  doesn't affect the flow of the traffic at all or create other problems, and is much cheaper.

It also looks a lot like this: Fisher Freeway & Lodge Freeeway.  Some of the ramps; however, come together prior to merging with the mainline.

Yes the Fisher & Lodge interchange is quite strange, especially with how far away from the interchange the necessary exits to switch freeways are. That interchange appears to be a windmill-turbine hybrid. I do think the far exit spacing might be because of the proximity to the Fisher & Jeffries interchange. Speaking of the Fisher & Jeffries, the first divergence on the ramp from Southbound I-96 to Eastbound I-75 seems unnecessary as the ramp that would be a lefthand turn causes a lefthand merge with I-75 when the other ramp also has a ramp breaking off from it to eastbound I-75 that employs a righthand merge instead. It seems like the extra ramp leading to a lefthand merge should be removed to improve traffic flow.

[zzomtceo]

Of course, with left ramps, you have the interchange at the Ford and the Lodge.

I think a stack would be best suited to that interchange because of the lack of surrounding space, it would be far from advisable to use the level of curve necessary to fit a windmill or turbine into that space.