Pre-Signaled Roundabout

From OpenTTD

Railway junctions 3-Way junctions Basic 3-Way Line Merge Half Cloverleaf Half Spaghetti Compact 3-Way Compact 3-Way B-M Junction Ultimate 3-way 4-Way junctions Basic 4-Way Roundabout Cloverleaf Spaghetti Star Complex Star Branch-Merge Over-Under Tetrathorp Pre-Signaled Roundabout Advanced Roundabout Right-of-way Roundabout Enormous junctions Double Cloverleaf Dual Tetrathorp Dual Branch-Merge Quad Branch-Merge 8-Way Star Other junctions Right-of-way Depot Dual tunnels Right-of-way Lane Change Unuseful juctions Crossover 3/4ths junction Ultimate 3-way junction for 4 tracks

The Pre-Signaled Roundabout is a combination of ideas. The primary idea is the Advanced Main Line Depot which uses signals to give priority to trains travelling on the main lines. The second idea is your basic Roundabout. One of the flaws of the Roundabout is that it can only handle 2 trains at a time. This slightly larger version can handle 4 trains. Additionally, trains turning right do not need to enter the Roundabout, instead passing by on the outer paths. So even if 4 trains are in the inner loop, other trains can still make right turns. The second flaw in a Basic Roundabout is that there is no preference given to exiting trains or entering trains. Using the Presignal Hack First described in Advanced Main Line Depot, we are able to give preference to trains exiting the loop. This greatly reduces the chance of a traffic jam. (I'm pretty sure the only condition that will cause a jam is four left turning trains entering the loop at exactly the same time.)

[edit] Pros:

• Traffic jams unlikely.
• Trains can use the junction to make a U-turn.
• No hills.
• Smaller footprint than most 4 way junctions.
• Cheaper than other 4 way junctions.
• Simple to start, complexity added later.

[edit] Cons:

• Small chance of a traffic jam by large trains.
• Trains are still slowed by extra turns.
• Complex signals may be hard to get right.
• Large amount of larger trains cause traffic jams.

[edit] Variations

An additional benefit is that this is a junction that can be built very simple to start with, and then built up. For example, you can start with a square version of the basic Roundabout, and later on convert it into the Pre-Signaled version. This makes it very cheap to start with, and later when money is available the job can be completed.

This is a little harder to build, but seems to be easier to find room for.