It’s not a matter of safety. It’s about flow.


Criticism of the plan for a roundabout at what was the Blinker intersection on the Edgartown-Vineyard Haven Road continues. The current jury-rig — four stop signs — is an issue, largely because in busy seasons long backups on the state highway irritate and delay drivers.

Whether the roundabout plan is the right remedy remains doubtful for a great many Islanders, though how many cannot be accurately judged. Certainly part of the apprehension and distaste derives from several issues that need additional explanation.

For example, the key virtue of the roundabout, as described by its designers, is safety, but a review of the design and of the way the roundabout works suggests that the chief virtue is actually traffic flow. The roundabout allows throughput, and ancillary elements of its design attempt to minimize the safety hazards that result from traffic moving more briskly through the intersection.

Answers given recently to Martha’s Vineyard Commission questions by John W. Diaz of GPI/Greenman-Pedersen Inc. in Stoneham, design consultant on the project, reinforce the view that speeding up the traffic may indeed be achieved by installing the $1.5 million roundabout, but safety is unlikely to be enhanced for drivers, pedestrians, cyclists and others.

As Mr. Diaz explains it, “While there is still limited data pertaining to safety improvements associated with converting a 4-way or All-way Stop controlled intersection to a roundabout, there have been some recent studies including the 2007 NCHRP Report 572 and the more recent NCHRP Report 672 that indicate there is no statistical change in safety between a four-way stop intersection and a roundabout.”

That assessment was based on actual statistics for the actual intersection that is the focus of debate. If one prefers an analysis based on a predictive model, the outcome might be otherwise.

Mr. Diaz again: “However, the 2010 NCHRP Report outlines a methodology to predict the number and severity of crashes between various control measures including 4-way Stop and roundabouts. Based on the methodology … as well as the historical crash data and volumes at the study intersection the following safety improvements are anticipated …”

The table accompanying Mr. Diaz’s letter shows that, given real data and employing the “methodology,” one might anticipate total anticipated crashes numbering 4.24. Of those, the total anticipated with injury might be 1.06. With the roundabout in place, non-injury crashes might be 3.46, or .77 crashes (about three-quarters of a crash) fewer. That’s a decline of 18.3 percent. And with the roundabout, crashes with injuries would number .42 (less than half a crash), or 60.2 percent, fewer.

In addition, Mr. Diaz forecasts that pedestrians may be safer because of the reduction of potential contact points between drivers and non-drivers, when the roundabout is compared with the 4-way stop. Maybe, but it’s awfully hard to see how contact between drivers and non-drivers is made safer by keeping the drivers moving smartly along and having the non-drivers dancing for safety among them, when, at the 4-way, all the drivers are stopped at least momentarily.

In light of this analysis, it seems reasonable to argue that the roundabout will keep traffic flowing more briskly. But to argue further that safety will be enhanced, especially when the intersection as it is does not seem especially dangerous anyhow, is at best fanciful.

Then there is the matter of safety, convenience and efficiency for Vineyard Transportation Authority buses and their passengers. The intersection is a transfer hub, and so bus activity must be accommodated in the design. That requires space adjacent to the roundabout itself, a good deal of space.

This tension between traffic flow and safety considerations means that this construction project will be extensive as well as expensive, involving easements to enlarge the right of way and a year’s worth of construction spread out over a couple of years, because no work will be done during the summer.

In addition, the roundabout will require some driver education. Roundabouts are uncommon, and drivers may be unfamiliar with the safe techniques required to navigate such a traffic artifice. Four-way stops are common and well understood by all.

The roundabout plan may be the right plan — economic and fluid — but the case for the wisdom of it has not been made.