I voted no - I stand divided on the issue to some degree, here's why:
1. Anyone who has travelled at 60 mph through southern Idaho, or Montana or eastern Wyoming or North Dakota for hours on end is probably the most excruciating thing I have ever experienced. The mind-numbingness of such an undertaking was in the interest of better fuel economy, but arriving many hours later at your destination does not offset the savings. In fact I figured that 60 minutes longer at work paid for the gas I would have saved.
2. Cars these days are much safer than decades ago when 100 mph was actually unsafe - even on a straight road. Modern cars can go faster, and the driver often feels like the car could and should go faster.
However! These arguments only apply to cross-country drives with low traffic density. Once you want to go 100 mph on, say Seattle freeways, then the sheer volume of traffic makes this just plain dangerous. Instead, urban freeways should have lower speed limits to discourage the illusion of "high-speed interconnects" between your home and work via automobile. Would you like to commute to work if you knew you'd be breaking the law if you went faster than 40 mph on the freeway? (The fast that with congestion sometimes the average speeds are quite close to that is subject to another thread...)
Have low speeds in cities and urban areas where trains whiz by at twice your speed, and high speeds links starting many miles outside of the city limits to enable quick transit between urban areas.
Who knows, here in the western USA this could turn the spark of the idea of "Park and Ride" into a real blazing boom!
So a national blanket speed limit, IMHO, is too simple. Lower the speeds in cities to encourage public transport and reduce accidents, and increase the limit (or get rid of it - Germany FTW) to enable high speed personal transportation between far away urban areas. (Really, I'm just thinking of the USA/Canada here.)
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