When transporting hazardous materials, how must you check your dual tires:

A At the start of each trip, when you park, and each time you stop.

explanation

Make sure your tires are properly inflated. Check placarded vehicles with dual tires at the start of each trip and when you park. You must check the tires each time you stop. The only acceptable way to check tire pressure is to use a tire pressure gauge. [CDL Handbook]