Earlier quoted context omitted.
I can't see how a 20 km/h difference can't not make a difference averaged over so many commuter-miles, but I'm not a city planner or traffic engineer.
You don’t need to be either. Suppose a trip is 5km. At 50km/h, that trip takes 6 minutes. At 30km/h, that trip takes 10 minutes. In practice, this naive way of calculating this doesn’t even reflect reality, because odds are the average speed of a driver through Helsinki was around 30km/h anyways. Going 50km/h between red lights doesn’t actually make your trip faster.
Except when it does, due to horrible traffic engineering practices.
There were a pair of one-way streets in the downtown of my city. Both attempted to have "green wave" setups for the lights. One worked pretty well, the other was okay, but whatever.
The problem was that the road itself was signed at 30 mph, but the lights were timed at 40 mph. It literally encouraged people to speed if it were not too busy (e.g., after business hours).