Diverging Diamond Interchange

THIS INTERCHANGE REQUIRES TRAFFIC MANAGER (and a version of NEP). To make it work like in the video, we’re going to have to set up a timed lights system.

Don’t be scared off though, it’s absolutely worth it. This type of interchange is can handle pretty much all your highway-surface street connections. It’s small, cheap, takes up almost no space, and is only two levels, and no steep grades, making it perfect for an efficient, high densinity, realistic city. This, or something like this, should be your go to for a service interchange, all it takes is a second to set up a coordinated lights system. Don’t worry though, it’s really simple for this interchange.

In fact, setting one of these things up is a great way to get familiar with traffic manager. Players complain about lights in this game. In real cities, carefully laid out, they’re what keeps traffic moving. With TM, we can do that. Not only are the results far better looking than building fly-over ramps and roundabouts all over the place, it really is a far less time consuming way to manage congestion.

As phased interchanges go, the DDI is pretty easy. Basically what’s going on is that one direction of the minor road will be allowed through at a time. That way it can cross the lanes going the other direction, to the left side of the bridge, where it can turn left turn onto the highway unobstructed, and then back to the right side of the road at the other end of the interchange. Just two phases.

It could work with just two lights, but we’re going to set up four. The reason for this is to eliminate weaving. When the lanes cross, it also allows drivers to take an unobstructed left off the highway, onto the minor road, but we want them to do this when those lanes are clear. We don’t want them to have to navigate through traffic travelling across the bridge, or taking a left onto the highway.

STEP 1: WHERE TO USE IT

The DDI is a service interchange, it’s for moving traffic onto or off the highway. Its capacity makes it perfect for providing highway access to high density residential or commercial districts. Also, the small size means it won’t get in the way. If needed it can always be upgraded to a DCMI later, which is a similar design with multiple grades instead of lights.

STEP 2: LANE CONNECTOR

The first thing we’re going to do in Traffic Manager is use "Lane Connector". We’re going to tell drivers exactly which road they’re supposed to be turning onto and which lane they should use. Just click on a node, then connect the dots to from approach lane to destination. As you do so, the turn markers on the pavement will change as well. I would do this immediately. Cims, as best as I can figure out, check your lane preferences when they start their journey. They won’t listen to you if you change it while their traveling.

The video I attached from U-DOT is a great way to familiarize yourself with this interchange. I also attached a picture of my lane connections. Here’s a description of how we’re going to drive through this interchange as well…

To cross the interchange from one side to the other, drivers start off in the center lane as they approach, which becomes the right of a 2 lane highway segment as the lanes are crossed, and then they’ll head into the bridge in the right lane. Keeping to the right during the cross over, they can enter the 6 lane road on the other side in either the center or left lane.

Drivers leaving the highway will exit from the far right lane into either lane of the two-lane highway ramp. If they are headed right, they do so keeping to the right. If they are taking a left, they will do so onto the center lane of bridge. They will stay in that lane as it turns into the left lane of a 2 lane highway through the crossover, and then into either the left or or center lane of the minor road.

Drivers entering the highway, if they are taking a right, will do so from the right lane as they approach DDI, onto the highway ramp, and then keep to the right as they merge. If they are taking a left onto the highway, they will approach the intersection in the left lane. From there they will proceed through the cross over, still in the left lane, and then into the left lane of the bridge, exit onto their ramp, and enter the highway in the right lane.

You may notice that these paths cross over each other, entering or exiting the highway by taking a left turn. Don’t worry, they will be doing so during different phases.

STEP 3: TIMED TRAFFIC LIGHTS

Okay, this is pretty easy, and a lot less labor intensive. We’re going to select four lights, and set up two phases for them. We’re actually going to start by adding another two lights to this interchange. Right where traffic leaving the highway to take a left connects with the bridge, we’re going to need a light there. Add that for both lanes with "Switch Traffic Lights".

Now use "Timed Traffic Lights" and click on those two nodes, as well as where everything connects with the 6-lane road on either side.

Click "Add Step" and lets set up phase one. Pick one direction of the minor road. Cars travelling in that direction will have constant greens. Also, cars will get a green to enter the bridge of the minor road heading the opposite direction. Cars turning right onto either direction of the minor road will also always have a green, in either phase. All the other lights should be red.

Set the time for something relatively short, I think I set mine to 10-15. It takes two phases for cars to turn left off of the highway, so we have to cycle them relatively frequently to stop them from overflowing the bridge. If there is ever a jam, this is how it will happen.

Now do the same for the other direction. Double check everything, make sure there are no lights that are always red, although several will be always green. Make sure each direction can proceed from one side of the interchange to the other on its phase, and that traffic entering the bridge from the highway isn’t doing so while through traffic isn’t using it.

By the way, one thing you may notice is that traffic taking a right turn onto the highway is going to have to wait at a red. It will get to go the same time the lanes next it do. That’s different from how DDI’s are normally done (like in the U-DOT video), but that’s to prevent traffic from having to merge as it enters the highway. Also, it keeps traffic entering the highway at a more constant rate, which reduces congestion on the highway. You could upgrade the on ramp to a 2ln highway if you want. That works perfectly well too.

Also, one slightly unrealistic thing is that you’ll notice one or two cars may be stopped by a light right before it enters the bridge, after it’s completed the cross-over. You could make that light always green if you wanted. IRL that would be both superflous, and incredibly unsafe. I let that happen just to be absolutely certain I catch that tail end of traffic approaching on the minor road before it enters the bridge so that it doesn’t interfere with the other phase. You could also use a transition phase, just to make the yellow a tiny bit longer, or just not worry about it. Honestly it doesn’t really matter.

STEP 3: I hope it’s working for you. If it isn’t and you can’t figure out what’s wrong, leave a comment here with a description and possibly a link to a picture, and I’ll try to help.

If it is working, take a moment to watch it from above. This is your city, these are your cims, you are their God. Today, you are a just and kind God. Where there was congestion, now there is motion. Though those beneath you may not have the capacity to comprehend your benevolence in its full glory, know that they appreciate it in their own little way. 🙂