I like and do most of my riding in the US mid-Atlantic region. It's humid here, and there isn't that much variation between daytime and nighttime temperatures. If it's hot, I wear riding shorts; if it's cool, I wear tights.
Now I'm planning to be part of an organized tour in Washington State in August, along the Olympic Peninsula. The organizers highly recommend that riders have some way to change their clothes during a day's ride to accomodate temperature swings. They even will pick up your arm&leg warmers from early rest stops and carry them to the day's destination for you.
Over the weekend, I tried on leg warmers for the first time. I have enough problems getting used to a single layer of lycra and just one elastic/silicone gripper on my thigh -- the thought of having two layers there for a substantial part of the day leaves me unimpressed.
So, what else have folks used for a leg-layer to shed as the day gets warmer? Would something like yoga pants or tights over cycling shorts work?
(And don't even start with arm warmers. But there, I know I can use a jacket or a bolero.)