we usually have the idea that when pedestrians (at all) are 'present', the maximum vehicle travel speed should be equal to the travel speed of the pedestrian. people walk at about 5.5 mph when they " have a purpose" (like heading to the bathroom).
about the driver's point. they miss the idea that the overhead guard is the reason they have an operator protection system [of which "seat belts" is a component] The goal is not 'to hold them on the truck when it stops from 40 mph', like in a car. The OHG will be moving faster than they are, when it runs off into the water, and will be what harms them. They would be better off with inflatable vests all the time when operating near the water, imnsho.