Simon, in the LA/Orange County area, that can be anywhere from 70mph to 0mph and everything in between. Usually multiple times in each trip.
I had a 100 mile commuting round trip for a while, and hated it. Then I went from that to a 200 mile round trip and after 3 months swore never again. As both were to and from Palm Springs, neither had much in the way of slowdowns, either. If there had been LA-type jams, no doubt I would have ended up in a rubber room.
And back in the days when real estate was booming, and people were flooding into the area by the tens of thousands, I saw a 28-mile commute go from 30 minutes to 40 minutes in the span of two years.