I'm not so sure about the speed limit being rigorously enforced on MacArthur. At least in the 6 years that I've looked down on it from my kitchen window...I see way too many cars ignoring the 25mph limit. That said, it is a road that has a relatively high volume of cyclists - especially on the weekends - so, at least for motorists who are familiar with it, there is some expectation of encountering them on the road.
Canal Road from the Foxhall/Macarthur interstection into M St. is one I'd recommend against, even for experienced riders. The speed limit is 35, but without anything on either side of the roadway, motorists often treat it as a mini highway and drive at 45/50mph. That's when its not bumper-to-bumper all the way from the Key Bridge to Foxhall...
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