At the moment, unfortunately, the required status and the optional status for paths are identical in effect. We will try to fix this later, but it is not so straightforward as making the required status for buildings work. However, telling the optimiser a pipe exists is possible, and the model will use existing pipes so long as they are cheaper than an alternative otion.
If you want to model existing pipes, you can tell the model two things, which are
- That the pipe already exists. This will make the pipe cost nothing in the optimisation, which should make its use more likely.
- That the pipe has a given maximum diameter. This will prevent any pipe being put there that would be bigger
These things are both changeable by selecting a path or paths and pressing the e key to edit its properties.
Unfortunately there is also no way to explicitly represent the step-change cost if you wanted to replace an existing pipe. If a pipe exists, it cannot currently be refitted to a greater capacity than the limit set. You might be able to approximate the step cost by fiddling with the civil costs, or by drawing a second parallel path which connects at the same junctions and making that optional.