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Free-Flowing Diverging Interchanges

Started by Haljackey, November 26, 2011, 01:59:17 PM

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Haljackey

Section 15: Free-Flowing Diverging Interchanges


The diverging interchange is a new breed of interchange, and is still mostly in the theoretical stage of development (in real life...obviously). This guide will show you how to build a Diverging Windmill Interchange (DWI...no, no, no, not "Driving While under the Influence", my Canadian friends), which is an interchange where both thoroughfares diverge, or cross over themselves, with drivers temporarily driving on the "wrong" side of the road (right-hand-drive countries would drive to the left of the median, left-hand-drive countries to the right of the median). While some of these crossovers may be signalized in an actual interchange, this guide will assume that all crossovers are free-flowing, to show how this concept could be applied to SC4 RHW-RHW interchanges. This particular variation is the one off-ramp, two on-ramp kind, but can be inverted to create a two off-ramp, one on-ramp design, which replaces a left-side entering lane with a left-side exit lane. (In the case of left-hand-drive, this design must be inverted, visually speaking, to provide a one off-ramp, two on-ramp variation. Pay attention later to outbound and inbound lanes.)

This guide assumes that you have a fairly thorough grasp of RHW networks, particularly FLUPs (FLexible UnderPasses). If you're having trouble understanding how to make this interchange, try building a few of the simpler interchanges instead.

Advantages and Disadvantages: When Should You Use it?

A DWI is a very high-capacity, high-speed interchange, with...get this...no movement conflicts. The dreaded left turn is eliminated, as is any weaving. The greatest thing going for the DWI is all traffic leaving the freeway exits before any new traffic comes on, reducing congestion (unlike nearly all configurations of the Cloverleaf). Furthermore, unlike the Windmill, the DWI doesn't require fly-overs, and so will conserve some space in the corners, making more room for a dense urban setting. (Many variations are possible, with different distributions of the footprint, but for the sake of this guide, I opted to compress it along the sides of each freeway as much as possible.) The DWI is also a much simpler design than the Turbine, Windmill, or Whirlpool, saving space, pavement, construction costs, stressballs, etc.

Another advantage is that the ramps and exit lanes, as well as most of the FLUPs pieces (not the FLUPs ramps), can accommodate other transit passing over or under them, such as HSR, monorail, rail, roads and el-rail/tram lines, as all the ramps are on the ground.

That said, it can get a wee bit complicated to build, mostly because traffic is going in opposite directions than usual in parts. Care must be taken to make sure you place everything correctly; I've made the mistake of putting FLUPs ramps or on-ramps backwards because of the confusion. I suggest copying the above diagram (included instead of the wikipedia one because it includes directional arrows), or better yet, plotting the interchange on 20cmx20cm piece of paper; that way you'll have something to refer to if you get lost, and you'll have a better understanding of how this thing works.

Due to the lack of RHW6, 8, and 10 "inside" ramps, this interchange is also only useful for RHW4-RHW4 networks, without rather significantly increasing it's size. In a later guide, I will show how the larger RHWs may be accommodated with flyover ramps or FLUPs pieces, as part of a Diverging Partial Cloverleaf Interchange.

Also, due to the incredible pain it is to conceive, let alone construct a grade-separated diversion of RHW4 highways using elevated pieces, all through traffic and left movements must pass through a few tiles of FLUPs One-Way-Road (OWR). Only three, not including ramps (eleven including ramps) but it's still a bottleneck (no worse than an OWR tunnel though). Right movements are completely free-flowing.

Small footprint, high capacity, and ease of upgrade make this one of the best interchange designs in SC4 (and in real life too).




In addition, this guide will show you how to build a Diverging Partial Cloverleaf Interchange (DPCI), which combines two of the most efficient interchange designs into one: The Partial Cloverleaf, combined with diverging elements. In this interchange one highway has a crossover movement, with drivers temporarily driving on the "wrong" side of the road (right-hand-drive countries would drive to the left of the median, left-hand-drive countries to the right of the median). While this crossover may be signalized at-grade, this guide will assume that are grade-separated (free-flowing), to show how this concept could be applied to SC4 RHW-RHW interchanges. Because there is only one crossover movement, and the DPCI converts two movement which would be left-lane (right lane for left-side drive) to the right, meaning that unlike the Diverging Windmill Interchange, this one can be used for RHW4-RHW6S, 6C, 8S, 8C, or 10 crossings.

This guide assumes that you have a fairly thorough grasp of RHW networks, particularly FLUPs (FLexible UnderPasses). If you're having trouble understanding how to make this interchange, try building a few of the simpler interchanges instead.

Note that this design can be inverted, to change which two quadrants will be least affected by this interchange. Doing so provides no other advantages.

Advantages and Disadvantages: When Should You Use it?

A DPCI is a very high-capacity, high-speed interchange, with no movement conflicts. Left turns are eliminated, as is any weaving. Like the DWI, all traffic exits each freeway before any enters. The DPCI also requires very little room for right-of-way (that is, it has a very small footprint) in two quadrants, just like the Partial Cloverleaf.

Another advantage is that the ramps and exit lanes, as well as most of the FLUPs pieces (not the FLUPs ramps), can accommodate other transit passing over or under them, such as HSR, monorail, rail, roads and el-rail/tram lines, as most of the ramps are on the ground.

That said, it can get a wee bit complicated to build, mostly because traffic is going in opposite directions than usual in parts. Care must be taken to make sure you place everything correctly; I've made the mistake of putting FLUPs ramps or on-ramps backwards because of the confusion. I suggest copying the above diagram, or better yet, plotting the interchange on a piece of paper; that way you'll have something to refer to if you get lost, and you'll have a better understanding of how this thing works.

Also, due to the incredible pain it is to conceive, let alone construct a grade-separated diversion of RHW4 highways using elevated pieces, all through traffic and left movements from the RHW4 must pass through a few tiles of FLUPs One-Way-Road (OWR). Only three, at the bare minimum not including ramps (eleven including ramps) but it's still a bottleneck (no worse than an OWR tunnel though). Right movements are completely free-flowing. This guide uses extra FLUPs pieces to achieve a cleaner, as slightly more realistic look. (Though in reality, a diverging interchange would more likely use flyovers than tunnel underneath itself to achieve grade-separated diversions. Oh well.)

Exceedingly small footprint, high capacity, and ease of upgrade make this one of the most efficient interchange designs in SC4.

15.1: Standard Diverging Windmill Interchange
15.2: Diverging Partial Cloverleaf Interchange

Back to the Table of Contents

Haljackey

Section 15.1: Diverging Windmill Interchanges (Created by CrimsonPhoenix)


Diagram: Chlewicki, Gilbert, "Variations of the Diverging Interchange" ITE 2010 Annual Meeting and Exhibit, 2010, Vancouver, Canada. TRB of the National Academies, 2010. (Cool article; this guide is based on it.)


General Notes

While technically an RHW-to-RHW interchange, this may also be used to upgrade a failing Diverging Diamond Interchange, Avenue-to-RHW.

Preparing the Grade-Separated Junction

Right, on to actually making this thing. Below is a crossing of two complete RHW4 freeways (traffic heading in the "correct" directions). I've added some road stubs to demarcate the ground as a visual aid; they are not required.


Next, plop down some road stubs to demarcate one of the freeways. Place four 8-down hole digger lots in the configuration below. Note that they are just outside where the two RHW4 freeways met, and that they are all perpendicular to the demarcated RHW4 (N-S).


Now, drag enough road to satisfy whatever slope mod you are using, like so:


Delete the roads, and use stubs to ensure that a 4x2 area is flat in your new trench.


Leave the stubs, and now plop four 8-up ground lifter lots, perpendicular to your new trench, three tiles away (two tiles from the slope). Drag road as below:


Be careful. Drag that road one tile too far and all the road stubs will come flying up...you know, they probably don't need to be there, but I'm picky. And this is a touchy interchange, to say the least (more on that later).

Now that that's done, trash all the roads and stubs. Get-out the RHW4 ON SLOPE transition pieces, and place them as below. The yellow lines go on the outside, and direction of traffic is reversed from normal.


Because of how the ground was prepared, the transition pieces should connect to each other nicely, and also lower the terrain behind them a bit. This is a good thing! The shallower the grade, the quicker we get back to the original level, the quicker we can place entrance ramps. (That, and in this case I opted for a dash of realism, trying to reduce how many changes in grade must occur for each thoroughfare...well, ignoring the FLUPs tunnels, but that's later.)

Now, plop a couple RHW4 starter pieces, in the same configuration as the overpass, with traffic going the "wrong" way. Drag-out RHW4 underneath the overpass, and delete the starter pieces:


Exit Ramps

Okay, on to our exits. The first exits will be on the inbound lanes. That is, the lanes that are travelling toward the overpass. We aren't going to plop these. Instead, we'll use the override to get a Style-B "Inside" entrance ramp, which doesn't have a puzzle-piece. Start an override like this:


Then drag diagonally out from that stub:


And you should get this:


Now, see that bit of MIS going backwards? Don't worry about it. We'll change it later, but that bit is going to make this particular ramp a bit finnicky later, as it will keep switching directions to the wrong way. Just a warning; it's a side-effect of using the override instead of the Style-A Inside puzzle-piece, in order to save space.

Your results may vary depending on your slope mod, and if you changed the design of the overpass/underpass. Just get it flat. I can't stress this enough. Keep the ramp, and the RHW4 opposite to it, FLAT. FLUPs ramps are incredibly neurotic about being placed on level ground, so if there is even a hint of a grade change on your ramp segment, go back, level, try again, or put it farther out. Or, accept the change in grade, but make sure you've got level ground running next to it.

Now that you have one entrance, rinse and repeat:


Crossover Sections (Diversions)

Now, to build our crossover sections. Start by plopping five tiles of stubs along the RHW (as a handy guide to make sure you have room for FLUPs; alternately, you can place the FLUPs pieces first), then an RHW4 lane shift, to get your inbound road on the "correct" side of the road:


Plop a FLUPs OWR entrance ramp, and one piece of diagonal FLUPs-under-pedmall.


Plop a FLUPs OWR exit ramp on the opposite side, and another diagonal FLUPs-under-pedmall. Then just plop a FLUPs-under-RHW4 segment to crossover the opposing lanes. Plop an RHW4/MIS1 Type-A entrance ramp in front of the FLUPs OWR ramp. Yes, that is two entrance ramps, running on the same side of the road, in opposite directions. Wacky!


Repeat three more times, until you get this:


Now, take a break, get a cup of coffee, and congratulate yourself. The hard part is done. Now all we have to do is connect the ramps! :)

Final Steps: Connecting the MIS

Drag out MIS ramps toward what will soon be your off-ramp. This will help you in plopping the next pieces.


Now, plop an RHW4/MIS-1 Dual Splitter A so that it connects to your ramps, then an RHW-6S/RHW-4 Dual Splitter Type A Exit to join that to your freeway. Join it all up:


Add a transition piece (RHW4-RHW6) somewhere before the exit, and redraw the stray piece of RHW6. You can see below I added a lane-shift to the opposing lanes, as I'm trying to save as much space as possible. Clearly, this isn't necessary if the original freeways had medians.


Repeat three times more, and you will have something like this:


And there you have it. A fully functional DWI that will fit nicely into a dense urban environment in need of a high-capacity interchange!

Variations

Here is a demonstration of the above interchange, with the ramps inverted, so that we have a two off-ramp, one on-ramp configuration:


The key to making this variation lies in where we place the FLUPs ramps. In this configuration, the inbound lanes crossover with FLUPs, instead of the outbound lanes as in the above guide.

Haljackey

Section 15.2: Diverging Partial Cloverleaf Interchanges (Created by CrimsonPhoenix)

Diagram: Chlewicki, Gilbert, "Variations of the Diverging Interchange" ITE 2010 Annual Meeting and Exhibit, 2010, Vancouver, Canada. TRB of the National Academies, 2010. (Cool article; this guide is based on it.)


General Notes

When building this interchange, the diverging road should be the one that handles the least traffic, such as an arterial, or RHW4.

While technically an RHW-to-RHW interchange, this may also be used to upgrade a failing Diverging Diamond Interchange, Avenue-to-RHW, as well as a failing 4-way RHW interchange that has weaving movements, like the Cloverleaf (in which not all traffic exits each RHW before new traffic enters).

Construction: The Junction

Alright, so to demonstrate quite clearly how this diverging interchange can accommodate the larger RHW sizes, I've built an RHW4 meeting an RHW10:


Now, bulldoze the RHW4, plop RHW4 starter pieces to reverse each lane's direction of travel, and drag it out again, like so:


You'll see that I left some stubs before I bulldozed the original RHW4. This was to help in plopping the starter pieces. Bulldoze them now if you used them yourself, and put some MIS ground-to-elevated and elevated-to-ground ramps in their stead, taking care to make sure they are placed correctly, as below:


See the yellow lines? The direction of travel of the MIS ramps is opposite to the RHW4 pieces directly next to them. They're going the "correct" way, right-hand drivers on the right side of the junction, and left-hand drivers on the left side.

Ramps, Exits, and Entrances

Next, plop or use overrides to create a Style-B entrance ramp as close to the MIS overpasses as you can along your non-diverging freeway, in this case, an RHW10. Then join the MIS in a circle. Repeat this in the opposite quadrant.


On the other side, plop an RHW4-dual-MIS outbound splitter connected to your MIS ramp, and join the empty MIS ramp to your main freeway. (See how that takes up almost no room? Fantastic indeed.)


Next, we bulldoze a section of the diverged RHW4. Place FLUPs OWR entrance ramps at the junction as seen below, and join them to their respective RHW4 lanes.


Next we add RHW4 Style B "Inside" entrance ramps using overrides. (There is no puzzle piece.) Start your override along the corner of the MIS ramp circle, like this:


Then complete the override by dragging-out diagonally. The starter MIS pieces at the end will be facing the wrong way. Ignore this for now; we'll fix it later.


Next, plop two road stubs on either side of the same RHW4, before the entrance ramp. This will ensure that there is room for the FLUPs pieces later. Then plop an RHW4 lane shift, and you should have this:


Now we add a FLUPs OWR exit ramp, leaving a tile of empty space for the FLUPs tile we'll place later. Add a Style A Entrance ramp after the exit.


Draw out the MIS ramps a bit, and connect them together with an RHW4-dual-MIS outbound splitter. This should fix the error that the override created before on the inside ramp.


Now, connect the RHW4 back to your original freeway. I added a lane-shift here to save space, but you can opt not to if you want a median.


Now repeat all that in the opposite quadrant, to get this:


Time to complete the RHW10 exit ramps. Drag-out your RHW4 diagonally toward the large freeway, and flare it at the end, like below:


Add a MIS-to-RHW4 transition piece to the end, and then attach the MIS to your highway with an entrance ramp, like so:


Sidebar: In the case of an RHW6S, the dual splitter type B may look more appropriate, but using it will reduce the through traffic of the major freeway to four lanes, instead of six until it leaves the interchange. Whether or not this is acceptable is entirely up to player preference. (In general, it's not the realistic thing to do, but to each their own.)

Now to complete the RHW4 exit ramps. Start by plopping an RHW6S-to-dual-RHW4 splitter just before the lane shift:


Plop an RHW4-RHW6S transition farther back, and connect it all up, redrawing the errant 6S piece at the exit.


Final Steps: FLUPs

Plop diagonal and straight FLUPs-under-pedmall pieces in the configuration below:


There is plenty of straight space there for another FLUPs ramp and an RHW4-to-elevated transition piece, if you want to create an overpass over the major freeway instead of using FLUPs. I used FLUPs pieces because it eliminates unnecessary grade changes, which is something civil engineers try avoid in real life.

Finally, use FLUPs-under-RHW4 and FLUPs-under-RHW10 (or 4, 6, or 8; if using 6C or 8C, you must construct the flyovers mentioned above, as these aren't supported by FLUPs...I think...) to finish the FLUPs routes. If you get an error when placing the RHW10 pieces, destroy the MIS ramps, redraw the RHW10, and then place the FLUPs pieces. Replace the MIS ramps and redraw them.

You should, when all is said and done, have something like this:


HUGE SUCCESS! Now go get a glass of wine or something. You've earned it.