[Hanno Spoelstra]

Since from 1940 onwards the lowest bridge Class was 5, apparently there was no need to classify vehicles with a lower weight "impact".

By the book, a jeep was a to display a Class 1 sign, but since the lightest bridges were Class 5, apparently all vehicles lighter than 5 tons could display a Class 5 sign as none of them would be too heavy to cross the lightest (the Class 5) bridge in use at that time.

Quote from http://www.canadiansoldiers.com/vehi...lemarkings.htm:

The 1940 regulations designated Class 5, 9, 12, 18, and 24 bridges, while by 1944, regulations stated that bridge classes would include Class 5, 7, 9, 12, 16, 18, 20, 24, 30, 35, 40, 50, 60, and 70.

The number designated the maximum weight class of vehicles which could safely cross that type of bridge.

These vehicle weight classifications did not necessarily denote the actual weight in tons of the vehicle (though for tracked vehicles the numbers often coincided.) The classifications were instead derived from considerations such as axle loading, distance between axles, and impact factor.

Bridges were to be marked with signs indicating the class, as was the route on which vehicular traffic would take to the bridge (with sufficient notice given to drivers who would need to divert due to their vehicle's weight).

Vehicles were likewise to be classified and marking applied designating the load they would impose on a bridge. The classes were designated in multiples of one ton, with the minimum being 1. A motorcycle was classified as a Class 1 vehicle, as well, motorcycles and motorcycle combinations (i.e. with sidecar) did not have bridging discs.

Vehicle classifications were rated so that they could cross a bridge with the same, or greater, numerical designation. For example, a Class 12 vehicle could safely cross a Class 12 bridge or a Class 18 bridge, but not a Class 5 bridge.