While there are undoubtedly those who would love to see a 50 mph speed limit here in the United States, it simply wouldn't be practical anytime in the foreseeable future. The Dutch study assumes that time added to trips by such a low speed limit would push more drivers to skip trips or switch over to using public transport. In Europe, where high-speed intercity trains are common, getting off the motorways is actually a viable options. The study also foresees that, over time, people would move closer to where they work, reducing the need for longer trips. In addition to CO2, the overall changes would also reduce other pollution and congestion.
In the spread-out United States, though, where more people live in suburbs and need to commute, a 50 mph speed limit simply is not practical. In order for a change like this to happen here, it would need to be implemented gradually over time to allow society time to adjust, and we just don't have time for that.
[Source: Transport & Environment | Image: Wikimedia Commons]