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For your safety...
To keep someone from starting the truck and moving it while you're performing the pretrip.
First aid kit, fire extinguisher, safety triangles,
Garbage truck drivers are require to carry a pre-trip inspection book in their trucks. The pre-trip inspection should be filled out each day before leaving.
Pre-trip School bus drivers MUST perform a complete vehicle inspection daily before starting their route. This includes all fluid levels, tire, body and general condition. Operation of all lights, warning signals and equipment. Condition and security of all seats and safety equipment. A proper pre trip inspection will take no less than fifteen minutes to complete properly.
Yes
air or, more likely, oil are checked before a trip. Gas, too.
find problems that could cause a collision or breakdown
Valve clearance
Yes. It is a law applicable to ALL CMVs.
Keep it in your pocket.
For one, it's just good practice to maintain positive control of your keys at all times. The reasons would be the same as the reasons for not leaving your keys in your own car, although there are other reasons, as well. One of the most important ones is the safety factor. A proper pre-trip involves inspecting the motor - if you have a truck which is slip seated, or you have a co-driver, or there's any chance of someone else cranking that truck, you don't want them doing that while you're inspecting that area of the truck. You don't want them putting it into gear and driving off when you're around the tyres, or when you may be under the truck or on top of it or the load on your trailer.