Nerf road speed at traffic lights / stop signs.

[originalfoo]

Currently it takes quite a bit of effort, per frame per vehicle, to reduce vehicle speeds when approaching a traffic light, stop sign, etc.

Why don't we just nerf the road speed (amount depending on segment length) so vehicles slow for those junctions just based on road speed without us needing to do anything else?

This could help resolve issues such as #756, and later be extended to implement #668.

[Marquillotuca89]

Thanks a lot, will try it and see if it works.