Consideration of introducing an s/t-coordinate system

[clemenshabedank]

Describe the feature

From the side of some traffic participant modellers the request was raised to introduce s/t-coordinate systems because it would make certain modelling tasks much easier. Before making an implementation suggestion I would like to assess potential advantages and disadvantages of the concept, as well as discuss if it would make more sense on a lane or a road basis.

Potential advantages are seen in helping with the following tasks:

• easily determine longitudinal distances in road coordinates, e.g. to vehicle in front or to certain signals such as stop lines, traffic lights, traffic signs
• easily determine lateral distances, e.g. to vehicle to the left/right
• easiliy specify if there is an adjacent lane not all the way adjacent

Of course all those calcuations could be done with x,y,z coordinates but it is more difficult and many simulators have s/t coordinates anyway.

Potential disadvantages:

• more data to be transmitted (but much could be configured to only be sent for use cases where needed, also pre simulation run)

Furthermore the s/t coordinate system could help in referencing a road surface description.

Describe the solution you would like / Describe alternatives you have considered

I could imagine two solutions and I don't know yet which is the best:

• Extend an s-coordinate to each lane, i.e. to lane boundary points (or centerline points, if this concepts stays in 4.0). Here I can think of 2 issues: First, if the coordinate system is just the linear connection of points, not every point can be represented in s/t due to kinks:
  
  Could be solved if polynoms where fittet but then the question is which polynom and through how many points. The second issue might be, that the same s-value for two adjacent lanes would NOT mean that vehicles were next to each other, because the inner-radius of a curve is smaller.

• Introduce a data type "Road" and introduce an s-coordinate to each road. Here the first issue is equivalent while the second isn't there. Of course, some effort would be needed to define a road message.

Describe the backwards compatibility

Additional context