...Then summer and winter movement times should be the same in places with different summer and winter movement times.
The temperate forest I'm based in has an average winter temperature of 51°, and I've never seen snow. However, the surrounding forest terrain, in which winter temperatures range between 47° and 53°, has a summer movement time of 1.7 hours...
...And a "winter movement time" of a whopping 9.7 hours. This is to account for the "snow" that can't physically happen in that terrain's climate.
TL;DR: Terrain travel times should be based on temperature, not season
The temperate forest I'm based in has an average winter temperature of 51°, and I've never seen snow. However, the surrounding forest terrain, in which winter temperatures range between 47° and 53°, has a summer movement time of 1.7 hours...
...And a "winter movement time" of a whopping 9.7 hours. This is to account for the "snow" that can't physically happen in that terrain's climate.
TL;DR: Terrain travel times should be based on temperature, not season