As we all know the volume of traffic at peak times causes huge queues and delays. In offering a discount on vehicle excise duty, it may encourage goods vehicle to be off road during peak times allowing a more substantial flow of 'rush hour' traffic.
It benefits the companies as the driver would usually have to take a driving breaks any way, and when they do return to the road, the drivers themselves would find easier flowing traffic as 'rush hours' would be over and instead of being stuck in traffic causing congestion and burning fuel they would have more freedom of movement at off peak times.
It may save the expense of building new roads and extending existing motorways.
Motorway cameras are all ready in existence and therefore it should be simple to enforce! Those who are in the scheme and breach it without good reason, could be posted the relevant fixed penalty notice.