The law requires a load to be arranged and secured so that there is no likelihood of danger, injury or nuisance either to those on vehicle or on the road. This covers not only part of a load falling from a vehicle but also loose commodities such as sand being blown from the vehicle.
Driving at Work by the Health and Safety Executive
DfT Code of Practice on the Safety of Loads on Vehicles
European Commission: Best Practice Guidelines on Cargo Securing for Road Transport