Actually just looked at my first Bronco (Canadian Staghound) kit today, must agree the instructions are not clear as most.
That being said here are a few photographs of a Bailey bridge in use last year:
That being said I have not answered your question, yet a picture is worth a 1000 words!
Hope the photographs help a bit. Being a modular design I believe they tripled up the trusses to allow for normal road traffic and loads over the span.