Improvements

[Corkonian]

Just add a speed sensor - reed contact and a magnet - to the setup to detect if the bike is moving. This could be used to detect downhill without pedalling or rolling towards a traffic light or standing... Would be only a few lines of code. Store actual speed, compare newe speed. Speed same or increased -> downhill, same or lower -> rolling towards a light. No crank, no speed -> standing.
Also, possibly decouple servo and actual pull at the bowden. Servos don't like constant pressure pull.

[Jan Oelbrandt]

Just add a speed sensor - reed contact and a magnet - to the setup to detect if the bike is moving. This could be used to detect downhill without pedalling or rolling towards a traffic light or standing...

Yes, that would be an easy and useful addition. The original script and setup was kept as simple as possible.
Add it yourself, and keep us posted on how it workes out!

Also, possibly decouple servo and actual pull at the bowden.

hmm... That would be great, how would you accomplish that?