How many wheel chocks you should use per trailer depends on several variables. Consider the following:
- Location and terrain
- Composition of area (asphalt, gravel, mud)
- Grade or slope, if any (front to back, side to side, or both)
- Is the trailer attached to the truck/tractor?
- What is being loaded on it (a ton of feathers vs. 20 tons of steel)?
- Is the trailer loaded/unloaded with a powered lift truck?
- Type of "parking brake" (air, hydraulic, or spring mechanical)
- Is the driver with the truck throughout the process?
- Time of year and time of day
- Weather conditions (rain, snow, ice, wind)
- Are you maintaining or repairing the trailer itself?
With so many variables, the answer is rarely simple. So, let's review a specific example to get a better idea of how to evaluate the situation and decide how many chocks to use.
You are in a highway tractor/trailer and are parked at a dock waiting to be unloaded at a warehouse by a powered fork lift and operator. You have set the brakes on the truck/trailer.
(Find out How to Design Safe Loading Docks and Warehouses for Lift Truck Drivers.)
How many wheel chocks do you need to use?
You need to use enough to prevent the trailer from moving, rolling, or tipping. Consider this from OSHA
1910.178 (k) (1) and (2) and 1910.178 (m) (7): "[t]he brakes of highway trucks shall be set and wheel chocks placed under the rear wheels to prevent the trucks from rolling while they are boarded with powered industrial trucks."
Other sources for guidance and regulations are the Federal Motor Carrier Safety Administration (FMCSA) and US Cargo Control. There are rules and regulations for each state and province that, while mostly standardized, are not all the same. You will also want to consider the company you are hauling for and the company you are delivering to or picking up at, as they will likely have their own standards and policies for chocking your wheels.
From a review of this subject for this answer, and from my past experience with CN Intermodal, it is totally dependent on the situation and jurisdiction. There is no standard answer, even when it comes to enforcement (in the USA).
Again, sticking to the specific scenario, you need two under the rear most wheels, or you need four – one under each of the rear wheels.
In my opinion only, based on how the OSHA language could be interpreted, I would say to use four wheel chocks, unless the warehouse tells you something different. And remember please, even if somebody says they will lock the trailer to the dock, set the brakes and chock the rear wheels of the trailer.
Chocking a trailer is a bit like tying up a boat. It's always the thing to do.