I strap a can of Pit Stop under my seat. Best case it costs me 30s to blast it into the tire and start riding again. Worst case I have to call the phone number for mechanicals/medicals and tell them I'm stuck at km whatever. Most people don't carry anything, as the chances are very low anyway, so it's best to not worry about it. Stopping to change a tire or tube would be pointless as you've already lost anyway.