interesting roundabout(ish) interchange proposed (Idaho)

[johndoe]

I can't remember seeing this layout before...


more info here:
http://www.southjeromeinterchange.com/
http://apps.itd.idaho.gov/apps/d4/Proposed_South_Jerome_Intechange.pdf (this shows higher detail)

I'm assuming all approaches are yield on entry, but not sure.  (No spiraling lane markings...will the outside lane of the arterial yield to circulating traffic?) It's a nice way to minimize bridge area, and it does give more space between the ramp intersections and the frontage roads. 

I wonder how many people trying to make a "left" from arterial to interstate will go around the roundabout and then discover they were supposed to continue straight to the signal and THEN make a left.  Oops!

Of course the big concern is probably the distance from the signal (with two phases opposing the "exiting" roundabout traffic...hopefully those frontage roads never attract lots of traffic).  Notice the lanes leaving the roundabout open to the left so lefts onto the interstate and lefts from the interstate will be occupying the same lane at those diverges.  Hopefully queues from the signal won't cause rear end crashes there!

[fillup420]

This looks like it will take up so much space that it will only be practical in places with little-to-no development or terrain.

[SD Mapman]

This looks like it will take up so much space that it will only be practical in places with little-to-no development or terrain.

Like Idaho!

That particular interchange already takes up more space than your normal diamond due to the angle of I-84 and S. Lincoln St. though.

[jakeroot]

Very British. I like it! Can't remember the last time the US built a giant roundabout like this. Somewhere in Massachusetts maybe?

[tradephoric]

It kind of reminds me of Weston Street/I-95 interchange outside Boston.  Although with the Boston interchange some of the on-ramps are from inside the circle

https://www.google.com/maps/@42.37127,-71.26988,544m/data=!3m1!1e3

[english si]

Very British.

It really isn't.

A British one would look more like this - as much as possible on the roundabout rather than putting stuff on side junctions (though there are some), the number of lanes on the roundabout carriageway being more constant with less channelisation, etc. Here's another example of how the Brits would do something like this (it has been changed recently) - rather differently.

[hotdogPi]

A British one would look more like this - as much as possible on the roundabout rather than putting stuff on side junctions (though there are some), the number of lanes on the roundabout carriageway being more constant with less channelisation, etc. Here's another example of how the Brits would do something like this (it has been changed recently) - rather differently.

Those look perfectly normal for Massachusetts (except for which side of the road people drive on).

[WillWeaverRVA]

This seems somewhat similar to the Shirlington interchange on I-395 in Arlington County, Virginia:

https://www.google.com/maps/@38.8403973,-77.0851925,594m/data=!3m1!1e3

[jakeroot]

Very British.

It really isn't.

A British one would look more like this - as much as possible on the roundabout rather than putting stuff on side junctions (though there are some), the number of lanes on the roundabout carriageway being more constant with less channelisation, etc. Here's another example of how the Brits would do something like this (it has been changed recently) - rather differently.

When I say "very British", I mean that its a huge roundabout-like interchange with two bridges. Speeds in the roundabout would be much higher than a typical American roundabout.

If this was being built in Australia, I could see describing this as "British" being a bit too far, since there are many other standard British-like roundabout interchanges all over the place (and there's enough differences with this design for that description to be somewhat incorrect). But, in the US, interchanges even laid out similar to this are exceedingly rare. So my tendency would be to compare it to the large British roundabout interchange (even if there are a few operational differences).

[english si]

When I say "very British", I mean that its a huge roundabout-like interchange with two bridges. Speeds in the roundabout would be much higher than a typical American roundabout.

For sure it's more British-like than what's usually found in the US. The idea I'm opposing is that it is 'very British' as if this is how we do it - as you say you would make note of the differences if it was in Australia.

It's as if you see someone drinking green tea and said "very British".

[kalvado]

When I say "very British", I mean that its a huge roundabout-like interchange with two bridges. Speeds in the roundabout would be much higher than a typical American roundabout.

For sure it's more British-like than what's usually found in the US. The idea I'm opposing is that it is 'very British' as if this is how we do it - as you say you would make note of the differences if it was in Australia.

It's as if you see someone drinking green tea and said "very British".

Would you be more comfortable with "somewhat British" or "having some British flavour"?

[english si]

Would you be more comfortable with "somewhat British" or "having some British flavour"?

No problem with either of them.

[lordsutch]

I'm assuming all approaches are yield on entry, but not sure.  (No spiraling lane markings...will the outside lane of the arterial yield to circulating traffic?) It's a nice way to minimize bridge area, and it does give more space between the ramp intersections and the frontage roads. 

On this diagram if you zoom in you can make out the "shark teeth" for yield lines on each of the four entry locations:

http://www.3pvisual.com/projects/jeromeinterchange/images/19338_Public%20Meeting%20Exhibits-Overall%20Exhibit_8.5x11.pdf

Of course it's not very "British" - today Highways England probably build it as a dumbbell with the frontage roads entering each of the roundabouts, ensure it was built with the minimum capacity such that it would barely function as of the construction year (much less the design year), and probably figure out some way to throw in a few peak-time signals for the lulz.

Actually, that's not fair to HE. Central government would probably make the local council build it, who would ask a developer to pay for all of it in exchange for planning permission to build a new housing estate (subdivision) with an Ikea nearby, and they'd slap a signalized roundabout right in the I-15 mainline instead.

[sparker]

This looks like it will take up so much space that it will only be practical in places with little-to-no development or terrain.

Which begs the question:  why build such an elaborate interchange (although both the entrance ramps to I-84 remain quite conventional) out in rural Idaho (Jerome residents, please refrain from throwing things at me!)?  Unless it's something of a "prototype" for future deployment (I can see the possibilities of something similar in the more built-up Treasure Valley area), it seems to be overkill. 

[Hurricane Rex]

If I was the developer of this project, I would put it at the Twin Falls exit otherwise I don't see where it is useful like you brought up sparker.

[jakeroot]

This looks like it will take up so much space that it will only be practical in places with little-to-no development or terrain.

Which begs the question:  why build such an elaborate interchange (although both the entrance ramps to I-84 remain quite conventional) out in rural Idaho (Jerome residents, please refrain from throwing things at me!)?  Unless it's something of a "prototype" for future deployment (I can see the possibilities of something similar in the more built-up Treasure Valley area), it seems to be overkill.

Here's the reasons, given in an ITD PDF:

The first one seems to the key here. Separate the off-ramps from the frontage roads.

[sparker]

It seems that Jerome is intending to position itself as a commercial/warehousing center, being next to I-84 (whereas Twin Falls, the regional central city, is across the Snake River gorge and thus limited in its access to the freeway).  In that context, what they're trying to do with the interchange makes a bit more sense -- the ability to move multi-trailer trucks easily & safely around the area would greatly enhance the commercial attractiveness of the immediate vicinity.  It'll be interesting to see if Jerome's population spikes in the next decade once relatively dense development commences.