Good question. I am not sure where I originally found the idea that shorter commutes make you happier, but I suspect it might have been an earlier version of this from 80000hours, which cites a couple of studies. Googling for pre-2013 media articles shows a lot of mentions of the idea as well.

The idea about a well optimized train or bus ride that Dr_Manhattan brought up also makes sense, if you live in an area with decent public transportation. It's the car drives that are a big time-killer, since you can't really turn your brain off while navigating through traffic, and traffic is usually more stressful at times you need to get to work.

There are a few productive things you can do during long drives though. For example, you can practice speeches, elevator pitches, songs, comedy bits, and so on without anyone hearing. That may not be quite as effective as interacting with another person on a bus/train, but the lack of an audience/consequences can make it easier to try out new things. Also, there's the option of consuming audio content (which you could also do with headphones on the bus or train).