Nodes, Segments, Lanes
Overview
In Cities: Skylines, vehicles and cims travel along routes (roads, tracks, etc.) which have been placed on the map:
Routes consist of one or more Segments
Segments contain one or more Lanes
Segments are joined together by Nodes
This article explains some of the nuances and terminology relating to them.
Segments
Each route consists of one or more segments, joined by nodes. Each segment has a node at both ends, even if not joined to other segments, and may curve and/or elevate between those two nodes.
Depending on the asset, there will be several segment styles ("elevations" / "prefabs") available:
Ground (G) - segment follows or flattens the terrain
Elevated (E) - small raised section or short bridge
Bridge (B) - robust bridge, can span long distances
Slope (S) - transition between tunnel and other elevations (i.e. it's the tunnel entrance)
Tunnel (T) - fully underground
Lanes
Segments contain one or more Lanes, on which vehicles can travel.
Lane Changes cannot occur in the middle of a segment; they can only occur at nodes. While road vehicles can change lanes at each node along a route, tracked vehicles can only change lanes at junctions and stations.
There are three common lane configurations within segments:
One-way - all lanes travel in one direction
Two-way - equal number of lanes in each direction
Asymmetric - different number of lanes in each direction
Nodes
A node is a point on a route between one or more segments.
Ghost nodes
Nodes should always be connected to at least one segment. However, due to issues with some mods (like old anarchy mods), it's possible to get nodes that aren't connected to anything. These are known as Ghost nodes (sometimes also referred to as Orphan nodes).
Ghost nodes are problematic because mods assume there will always be a segment attached; this can cause crashes or annoying Array index errors. Use Broken Nodes Detector mod to remove ghost nodes.
Terminal nodes
Terminal nodes occur at the unconnected end(s) of a segment:
Routes which end in a terminal node are called 'dead-ends' or ' cul-de-sacs'.
Roads (Terminal)
U-Turns can always be performed on terminal nodes
Pedestrian Crossings (no road markings) are always enabled
Tracks (Terminal)
Trains can't use dead-end tracks
Any train on a dead-end track will ultimately despawn
This also applies to monorail, tram, trolleybus, and metro
Middle / Segment nodes
Middle nodes (sometimes called Segment nodes) seamlessly connect two segments together:
Roads (Middle)
Lane Changes can occur
Pedestrians can join nearby pedestrian paths from these nodes
Tracks (Middle)
Trains must go straight-ahead at these nodes
This also applies to monorail, tram, trolleybus, and metro
Bend nodes
When two segments join at sharp angles, they sometimes create a Bend Node (depends on settings of the road asset):
At first glance, a bend node looks like a middle node. However, on closer inspection that's not the case:
The node acts like a virtual segment:
It copies the style of one of the adjacent segments
It's still a node, even though it looks like a segment
The two segments either side are shortened slightly to accommodate the virtual segment
Roads (Bend)
Lane Changes can occur
Pedestrian Crossings (no road markings) are enabled by default
Tracks (Bend)
AVOID!!
Bend nodes are likely to break the vehicle AIs
Junctions with two segments
they occur between segments of different kinds of road. Generally, if there are different number of lanes, or the other road is different size or elevation, you'll get a junction instead of a bend/middle node:
Roads (Two-Segment Junctions)
Lane Changes can occur
Pedestrian Crossings (with road markings) are enabled by default
U-Turns are disabled by default
Unless one of the segments is one-way; otherwise there is no way to reach the other side of the two-way road
Tracks (Two-Segment Junctions)
Every incoming lane will be connected to every outgoing lane of the other segment
Asymmetric nodes
Asymmetric nodes occur where asymmetric roads change direction (basically a stylized middle node):
Roads (Asymmetric)
Lane Changes can occur
Vehicles in the terminated lane must merge in to one of the remaining lanes
Pedestrian Crossings are disabled by default
U-Turns are disabled by default
Tracks (Asymmetric)
Currently, there are no asymmetric tracks available
They would likely behave the same as tracked transition nodes
Junction nodes
Junction nodes occur at the confluence of three or more segments of road or track (not both):
Roads (Junction)
Lane Changes can occur
Vehicles can change to any outgoing lane
U-Turns can optionally be allowed
Pedestrian Crossings (with road markings) are enabled by default
Vanilla: Traffic Lights and Stop Signs can be used for additional control
Tracks (Junction)
Lane Changes can occur
Vanilla: Stop signs can control right-of-way
Level crossing nodes
Level Crossings are Junction Nodes with both road and tracked segments; they can only occur at ground level:
Only one type of traffic can use the junction at a time; either road vehicles or trains. To enforce that, level crossings have barriers that block road traffic from entering the junction while a train is approaching or crossing the junction. The barrier will only lift once the train has fully cleared the junction (it's no longer on a tracked segment either side of the junction).
Roads (Level Crossing)
Lane Changes can occur
Vehicles can change to any outgoing road lane
Vehicles cannot cross the tracks while barrier is down
U-Turns are disabled
Pedestrian Crossings (no road markings) are enabled by default
Pedestrians cannot cross the tracks while barrier is down
Tracks (Level Crossing)
Trains must go straight ahead at the junction
I have no idea what happens if someone tries adding 3+ tracks to the junction
Nodeless networks
As mentioned at the start of this guide, segments always have a node at each end. Well, that's not entirely true.
Some workshop road and track assets provide Nodeless networks which allow segments to connect to junctions without the side effects of having a node:
The benefit is that the end of the segment won't be distorted by the junction node; this facilitates some innovative design opportunities for asset creators and detailers
The downside is that some traffic management features, such as stop signs and traffic lights, aren't available on the nodeless networks.
Nodeless Examples
The RWY project uses nodeless tracks to achieve much nicer-looking junctions (top is normal junction, bottom is nodeless):
The LRT project includes a sinking tram track, achieved by having two roads (an avenue road and a tram road) on top of each other:
The CSUR project offers pick-and-mix nodeless networks to achieve extremely flexible urban roads:
