Applied Horizontal Alignment

Design Speed, Curve Radius, and Hierarchy

Road Speed 1 Speed 2 Highway 100 km/h >80hm/h Ramp 80 km/h 50-80 km/h Loop 50 km/h 30-50 km/h

* taken from the Dutch Freeway Design Manual [publicaties.minienm.nl] (p. 2-4), adapted for Cities: Skylines use

Transition Curves

[en.wikipedia.org]

If such easement were not applied, the lateral acceleration of a [...] vehicle would change abruptly at one point – the tangent point where the straight [...] meets the curve – with undesirable results. With a road vehicle the driver naturally applies the steering alteration in a gradual manner and the curve is designed to permit this, using the same principle.

Loops and making it look good

Remember road hierarchy? We'll be using theand-type roads in the rest of this guide. and a quick and easy way to get the minimum curve sizes you'll want to be using with these is to look at their design speeds first.Again, in Cities: Skylines ashould have curves designed for a speed of 100 km/h, withcoming in at 80 km/h. As in real life though, these kinds of minimum design standards can't always be reached. Loops for example (or rather, >180° turns) would have to be made so large in-game that they become almost impossible to place.In these cases, it's best to go down a 'step' in road hierarchy. This doesn't mean that certain connectors should be made from roads with a lowerstandard; please don't start adding zebra crossings and such. However, a lowerhelps a ton in keeping road footprint to a minimum and in limiting interchange size as well. I've summarized the 'steps' you may take with regards to differing hierarchies in the table* below:When usingor even looping connectors, constraining maximum design speed is a totally legitimate way of limiting curve size (and therefore footprint). In real life, you'll often see hard or suggested speed limits on connectors for this exact reason. So if there are no other alternatives (like using more space), 'stepping down' the hierarchy ladder is something you'll probably have to do as well. In-game, like in real life, space alone comes at a price premium, and expansive designs cost a lot more money to create too.What this means is that, whereas an 80 km/h, 90° curve would need a whopping 44x44 cells in-game, it is totally cool to go down to a 50 km/h design speed and therefore a 16x16 cell curve. In general, smoother curves go up in radius exponentially: 50m for a 30 km/h one, 135m for 50 km/h,m for 100 km/h. At the bottom of this guide there's a handy reference sheet where minimum curve radiuses are translated to in-game cell dimensions. I've also added the 45° and 15° curve sizes for easy look-up (using basic trig and calculus to compute them). Use it when you need it!Going 'down' from a 100 km/hto, say, a 50 km/his not something that should be done in one go. In other words, a full-speed roadway should never be followed by aspeed minimum radius curve. Instead, drivers have to beinto the follow-up minimum design speed. I'll quote Wikipedia here for the explanation on this:An example: let's start off off with a basic 90° 20-cell curve (design speed is about 55 km/h):If this were to directly connect to a higher speed road, it would spell trouble for drivers because of too abrupt a design speed change. Instead, we need to compound or split it up to make it into a 'transitioning' one. We might for example cut it in half first, so that one 90° curve becomes two 135° ones. In order to do this, it's easiest to draw a line (or rather: a road) right through the middle of the original curve, and work from there:Now you can screw around with making these two compounded curves look best. Here I've gone for a pretty tight 11 by 6 cell @ 135° (design speed 45 km/h) one:And ended up finishing it quick and dirty by doing the same thing from the other side:As you can see in the second to last and final pics, what this gives you is two transition curves meeting at their shortest ends. Although the difference might not end up looking very big compared to the original curve (right and outside), design speed has gone down as a function of tightening the minimum curve radius. If you were to bring the speed back up again to 55 km/h, those 45° curves would need to meet in 8 cell-length parts.Let's compare:Both these curves have the same design speed, but now the elongated outer connector (made out of two 12c by 8c 45° transition curves) is the bigger one. As soon as you start to apply transition curves, you'll need more space!Another example of using transition curves is with loops. Below, I've shown two differing horizontal alignments for loops within a 24x24 footprint. Both of them have got a design speed of only ~40 km/h - pretty much unavoidable if you don't want them to end up being way too massive for the game - but the one on the right uses 18c by 12c 90° transition curves:As you can see, the loop on the right looks a bit more natural than the one on the left. Entering a 18c 90° curve, drivers arefrom between 55-50 km/h into a 40 km/h turn. In real life, ample space would be provided for drivers to slow downentering these curves too, by ways signposting progressively slower speeds for example (going down from 100 to 50 km/h maybe). In-game, you can kinda simulate this by having enough of a 'run-up' to the connector, so that ample warning time of the impending curve is provided for.In the end, doing this keeps transition curve length in check as well. When moving in towards a tighter radius in-game, it is generally, with a maximum of it being around twice as long at the 'faster' end. With this, it is easy to end up with good-looking connectors that go from, say, >40 to 35 km/h in tight radius 8c loops, or sweeping 90 to 80 km/h fly-overs (27 by 18) with the stack interchange at the top of this section.