Very cool. Unfortunately it looks like I managed to put it into an infinite loop in the tutorial. Or possibly I just didn’t wait long enough after saving my changes…
I haven't given any love to the poor tutorial mode in ages, so I'm not surprised you found a bug. Please file an issue (https://github.com/dabreegster/abstreet/issues) and describe how to reproduce the problem. If you can open the browser developer console (Ctrl+shift+I in firefox), often the last part will be really helpful.