Just for information I've suffered from heat syncope (fainting; who knew there was a word for it?)
I felt hot, but ok. I was walking and talking under shade. The next moment something grey was rushing towards me - the ground, and the next moment after that I was lying on a St John's Ambulance cot with a cold compress on my forehead.
Just a slightly awkward situation at a sporting event with a first aid room, but it could be catastrophic on the road.
So yeah, if you start to feel the negative effects of heat, it's probably a good idea to find some shade and water and get your core temperature under control.
(heat syncope is when your blood vessels expand to try to drop your body temp, and the sudden drop in blood pressure is followed by a sudden drop in consciousness)