Our roads are congested by large trucks. They are not only slowing down traffic, but due to the incredible weight disparity in comparison to personal vehicles, they create a lot of risks too.
Since the energy of the impact is exponentially growing with speed, large trucks should not be allowed to drive faster than the allowed speed limit minus 5 (or even 10) mph.
Beside the positive impact on high road safety, it would reduce the wear and tear on the roads, make the gas mileage better (reduce air pollution, dependency on foreign countries for energy needs, etc.)
The reduced speed would make transportation companies consider alternative transportation means such as the railway.