As a former commercial truck driver and someone that's toured many roads between
Fairbanks Alaska and Yuma Arizona, I prefer cyclists wear high visibility reflective vests
all the time, and good bright blinking lights in low light conditions. I always wear a vest
even in the heat of summer and haven't had any issues with heat buildup or discomfort.
Now helmets are another matter. Although I always wear one, I've had heat problems
during long climbs in the summer.