I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow designs

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jackal
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I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow designs

Post by jackal »

This interchange is NW of Champaign, IL and is presently a basic cloverleaf constructed in 1965. The report presents some interesting detail regarding five freeflowing interchange options.

Directional/semi-directional ramps were all designed for 50mph, loops for 40mph (requiring rebuild of the tight existing loops). Ramp travel time is summed across all eight ramps and is a function of ramp length and design speed.

Table 1 - Copy.JPG

Table 2 - Copy.JPG

Alternatives 1 (stack) and 2 (cloverstack) ranked highest and were taken forward for further development. At this point directional ramp speeds were increased to 55mph in line with new guidance. The comparison is largely as before except cost estimates increased to $170m for alternative 1 and $140m for alternative 2. Alternative 2 also ranked better in dimensions such as environment and was ultimately selected. The project is to be let in June 2021.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by Truvelo »

135MB is certainly one of the largest PDF's I've ever downloaded. Anyway, the yanks don't mess around when it comes to upgrading junctions. I dread to think what would be built over here. No doubt the initial proposals would be as grand but the eventual design would be something more mundane, probably just a direct connector to replace one of the loops.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by booshank »

Truvelo wrote: Thu Oct 01, 2020 16:10 135MB is certainly one of the largest PDF's I've ever downloaded. Anyway, the yanks don't mess around when it comes to upgrading junctions. I dread to think what would be built over here. No doubt the initial proposals would be as grand but the eventual design would be something more mundane, probably just a direct connector to replace one of the loops.
It would probably involve a roundabout or three, five sets of traffic lights and a minor road plugged in to allow access to a retail park and housing development. Oh and sold as an "upgrade".
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by booshank »

jackal wrote: Thu Oct 01, 2020 15:21 This interchange is NW of Champaign, IL and is presently a basic cloverleaf constructed in 1965. The report presents some interesting detail regarding five freeflowing interchange options.

Directional/semi-directional ramps were all designed for 50mph, loops for 40mph (requiring rebuild of the tight existing loops). Ramp travel time is summed across all eight ramps and is a function of ramp length and design speed.


Table 1 - Copy.JPG



Table 2 - Copy.JPG


Alternatives 1 (stack) and 2 (cloverstack) ranked highest and were taken forward for further development. At this point directional ramp speeds were increased to 55mph in line with new guidance. The comparison is largely as before except cost estimates increased to $170m for alternative 1 and $140m for alternative 2. Alternative 2 also ranked better in dimensions such as environment and was ultimately selected. The project is to be let in June 2021.
Interesting! I'm a little surprised they went straight from cloverleaf to a full upgrade rather than adding collector distributor systems, but maybe they thought that would be poor value for money for a temporary solution.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by booshank »

They considered a three level roundabout! But rejected it pretty summarily:-

Image

The selected design is pretty big compared with the existing cloverleaf. The loops are massive - I make them about 370 m from outermost point to where the mainlines cross. Probably about the same radius as those at Croft (but not as elongated).

Image
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by Truvelo »

It would appear the circle interchange is freeflowing rather than a conventional roundabout. It may work where turning traffic volumes are light.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by Peter Freeman »

Truvelo wrote: Thu Oct 01, 2020 20:32 It would appear the circle interchange is freeflowing rather than a conventional roundabout.
This roundabout shape (exits not co-sited with entries, but following a weaving length) is like early UK designs ('early' meaning late 1960's, after the introduction of universal give-way-to-right but before standardisation on circular central island). Especially common in the over-enthusiastic and mainly-never-built plans for urban motorways in the Buchanan Report period. However, even this shape provides radically insufficient weaving space. The theory was flawed.
It may work where turning traffic volumes are light.
Many poor interchange designs will work where turning traffic volumes are light.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by booshank »

Peter Freeman wrote: Fri Oct 02, 2020 00:50
Truvelo wrote: Thu Oct 01, 2020 20:32 It would appear the circle interchange is freeflowing rather than a conventional roundabout.
This roundabout shape (exits not co-sited with entries, but following a weaving length) is like early UK designs ('early' meaning late 1960's, after the introduction of universal give-way-to-right but before standardisation on circular central island). Especially common in the over-enthusiastic and mainly-never-built plans for urban motorways in the Buchanan Report period. However, even this shape provides radically insufficient weaving space. The theory was flawed.
It may work where turning traffic volumes are light.
Many poor interchange designs will work where turning traffic volumes are light.
Presumably a more conventional roundabout wasn't considered because of the lack of freeflow, but this thing was because it's freeflow at the cost of probably being more dangerous.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by jackal »

There are a few large freeflowing stackabouts in Europe, e.g.: https://www.google.co.uk/maps/@41.0104772,14.195848,16z
The weaving distances are comparable to this nearby cloverleaf: https://www.google.co.uk/maps/@41.0032339,14.008906,16z

Of course, a big difference is that the weaving is on the mainline in the cloverleaf and off it in the stackabout. In this respect a stackabout is comparable to a cloverleaf with C/D lanes - it has better geometry (no tight loops) but one extra conflict point (three rather than two) on the long turns (lefts in the US). To put that last point another way, a long turn on a freeflowing four-arm roundabout has to weave with all three other long turns, whereas on a cloverleaf it only has to weave with two of them (between the loops).

Of course none of the above can really be considered a modern design competitive with a cloverstack or similar.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by booshank »

jackal wrote: Fri Oct 02, 2020 11:21 There are a few large freeflowing stackabouts in Europe, e.g.: https://www.google.co.uk/maps/@41.0104772,14.195848,16z
The weaving distances are comparable to this nearby cloverleaf: https://www.google.co.uk/maps/@41.0032339,14.008906,16z

Of course, a big difference is that the weaving is on the mainline in the cloverleaf and off it in the stackabout. In this respect a stackabout is comparable to a cloverleaf with C/D lanes - it has better geometry (no tight loops) but one extra conflict point (three rather than two) on the long turns (lefts in the US). To put that last point another way, a long turn on a freeflowing four-arm roundabout has to weave with all three other long turns, whereas on a cloverleaf it only has to weave with two of them (between the loops).

Of course none of the above can really be considered a modern design competitive with a cloverstack or similar.
I think I'd prefer the cloverleaf. Not only are there two conflicts rather than three, but the weaving distance is straight rather than curved and joining and leaving traffic likely to be slower due to the geometry of the loops.
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Re: I-57 and I-74 Interchange Reconstruction - includes costs, land take, journey times & safety of various freeflow des

Post by jackal »

The roundabout weaving area is straight in the US drawing and a very gentle curve (200m radius) in the Naples example, so I don't think that's really to the favour of the cloverleaf. The loops slowing traffic is not really a good thing either as you have a risk of trucks tipping over, and it also means the weaving area is a mixture of slow but accelerating traffic coming off a loop and fast but decelerating traffic approaching a loop.
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