The US wants to force lower speeds on truck and bus drivers. The linked article explains that the government has proposed capping the maximum speed of large trucks and buses. This will be done by an engine limitation so that the driver cannot drive faster even if desired (or the safer course of action).
The reason is a reduction in fatal crashes -- the government estimates the reduction in fatalities if trucks were limited to a lower speed. They also promote the savings in fuel costs.
Of course, this is the government's side. Any fuel cost saved by driving slower will be offset (or perhaps exceeded) by extra wages to the driver. Or for independent drivers by lower incomes as they are paid the same amount but take longer to get to their destination. Assuming, of course, the driver doesn't drive extra time to make up for the lower speed (illegal but easier to get around than an engine limiter).
As pointed out briefly in the article, but apparently not part of the government's reasoning, is the increased accidents as trucks drive slower than cars. Many highway accidents are a result not of the absolute speed involved but in differences in speed between vehicles. Trucks limited to 60mph (one proposal) will have to watch out for cars driving a legal 75mph weaving in and out of lanes.
To those of us old enough to remember the 1970s and 1980s the lower speeds are nothing new. The government imposed a nationwide 55mph speed limit, ostensibly to save fuel and money. If truck speeds are going to be limited and there follows an increase in accidents due to slow trucks and fast cars, how long until speed limits are reduced? Auto makers could then be ordered to limit the car's maximum speed to help enforce the speed limit. Maybe Sammy Hagar's classic song "I can't drive 55" will top the charts again.