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In defense of rotaries
Jul. 1st, 2010 12:51 amI was a bit surprised, though I guess I shouldn't have been, to find this discussion of Massachusetts rotaries on wbztv.com that takes utterly for granted the assertion that rotaries are a dangerous archaism that should be removed wherever they exist.
As the author of Hell Rotaries of Massachusetts, which mentions some of the very rotaries being talked about here, I feel some small responsibility to speak up about this. (I also drive through some of them every day lately, so I've been thinking about this.)
The modern roundabout is, if anything, a superior alternative to signal-controlled intersections in many situations, and as far as I know they're slowly gaining popularity. In the Hell Rotaries post,![[livejournal.com profile]](https://www.dreamwidth.org/img/external/lj-userinfo.gif)
manfire mentioned an intersection in Towson, Maryland (his link is now dead) that was converted to one not long ago with a substantial improvement in local traffic.
The problem with these rotaries on Route 16 and environs is that, by and large, they're not modern roundabouts. A few of them come close; many don't. The key error is to have the entering and exiting routes meeting the circle almost at a tangent, especially if there's a lot of through traffic moving tangentially to the circle. Those cars are going to take the rotary too fast, and the drivers will often be unable or unwilling to yield to the traffic in the rotary, which is in theory the cardinal law of both Massachusetts rotaries and modern roundabouts. The result is that the rotary can contribute more than it should to gridlock and accidents. In general, you don't want to design a rotary to maximize the speed of the cars in it; you want the affordances to naturally slow them down a little, which can actually increase the throughput of the rotary by letting all the cars interact better.
The main complaint about rotaries, as in that discussion, is that "no one knows how to use them". But Massachusetts is one of the few parts of the US where there's actually some awareness of how to drive through a rotary. I'd hate to see all those rotaries on the Mystic Valley, Fresh Pond and Alewife Brook Parkways ripped out and replaced with signal-controlled intersections (there's too little space for anything else). It would turn the whole area into even more of a parking lot than it is. What they really ought to do is reconfigure those rotaries into modern roundabouts, which in some cases would just take a little repositioning of the legs of the circle.
(Of course, if the object is to discourage automobile traffic altogether, as in plans for walkable center cities, that's another story--but that requires viable alternatives, which don't really exist there in sufficient quantity.)
As the author of Hell Rotaries of Massachusetts, which mentions some of the very rotaries being talked about here, I feel some small responsibility to speak up about this. (I also drive through some of them every day lately, so I've been thinking about this.)
The modern roundabout is, if anything, a superior alternative to signal-controlled intersections in many situations, and as far as I know they're slowly gaining popularity. In the Hell Rotaries post,
manfire mentioned an intersection in Towson, Maryland (his link is now dead) that was converted to one not long ago with a substantial improvement in local traffic.The problem with these rotaries on Route 16 and environs is that, by and large, they're not modern roundabouts. A few of them come close; many don't. The key error is to have the entering and exiting routes meeting the circle almost at a tangent, especially if there's a lot of through traffic moving tangentially to the circle. Those cars are going to take the rotary too fast, and the drivers will often be unable or unwilling to yield to the traffic in the rotary, which is in theory the cardinal law of both Massachusetts rotaries and modern roundabouts. The result is that the rotary can contribute more than it should to gridlock and accidents. In general, you don't want to design a rotary to maximize the speed of the cars in it; you want the affordances to naturally slow them down a little, which can actually increase the throughput of the rotary by letting all the cars interact better.
The main complaint about rotaries, as in that discussion, is that "no one knows how to use them". But Massachusetts is one of the few parts of the US where there's actually some awareness of how to drive through a rotary. I'd hate to see all those rotaries on the Mystic Valley, Fresh Pond and Alewife Brook Parkways ripped out and replaced with signal-controlled intersections (there's too little space for anything else). It would turn the whole area into even more of a parking lot than it is. What they really ought to do is reconfigure those rotaries into modern roundabouts, which in some cases would just take a little repositioning of the legs of the circle.
(Of course, if the object is to discourage automobile traffic altogether, as in plans for walkable center cities, that's another story--but that requires viable alternatives, which don't really exist there in sufficient quantity.)