By Steven Inglima, Guest Columnist
The recent proposal for a roundabout as a traffic control method for the intersection of Peachtree Street, Tennessee Street, Hiwassee Street and Valley River Avenue – the main intersection in the middle of downtown Murphy – sounds wonderful.
However, examination of the area required for a successful roundabout compared to the space available at this intersection suggests it likely can’t work; there’s not enough room.
The success of a roundabout relies on several competing factors. The first is the available radius of the circle of the roundabout. The next is the volume and type of traffic, and the last would be the speed limit of the feeder roads.
The only successful and applicable factor is the 25 mph limit for the feeder streets. However, the other two factors fail to provide sufficiency.
Using the measurements of the intersection, we find that without chewing into the private property of the businesses that meet that intersection, there is a radius of a little less than 40 feet from center to what would be the outer circle of the roundabout.
Because N.C. Department of Transportation rules suggest a minimum of 12 feet for the width of a road, the inner radius for this circle would be about 28 feet. Thus the median circumference of this circle would be 213 feet or so.
Let’s pretend that the largest vehicle that would enter this circle is a typical auto, which averages around 14 feet. Keeping one car length between each vehicle in the roundabout would allow seven vehicles to simply be there. However, this does not factor the room it takes for a vehicle to safely enter when there are cars within
that circle; that would now demand less vehicles to safely be in the circle.
Let’s consider that we can’t restrict the traffic to family cars, and factor in the reality that trucks of various sizes have to use this same intersection. The number of vehicles that could enter or occupy the roundabout would be reduced perhaps in half.
Let’s also consider that there are two lanes of traffic for each of four streets that meet at this intersection. We know that it’s likely to see four to six vehicles in each of the lanes waiting at the light. The amount of delay we can anticipate with the restrictive radius circle would actually increase the time it takes to pass the intersection.
Now consider that pedestrians trying to cross in any direction will have to contend with a nearly constant flow of cars traveling perpendicular to their crossing. At least with a traffic control light, there is a break in vehicular movement that is predictable.
A good reference for this is the Texas DOT web page concerning roundabouts: http://onlinemanuals.txdot.gov/txdotmanuals/rdw/roundabouts.htm
They describe the common INSCRIBED circle diameter MINIMUM as 45 feet (R-22.5 feet), demanding the outer circle be 46.5 feet, not available in Murphy.
The National Associations of City Transportation Officials published the following on its website:
Mini roundabouts and neighborhood traffic circles lower speeds at minor intersection crossings and are an ideal treatment for uncontrolled intersections.
Note the specification of “minor intersection,” and also uncontrolled intersection. Yet, we might be contemplating this for one of the most “major” intersections in the city, currently controlled by a traffic signal.
Let’s take a look at a few of the roundabouts in our area. The roundabout in Hayesville has a radius of 77 feet. That’s almost double the available space in the center of Murphy, and it handles considerably less traffic.
The roundabout in Blairsville, Ga., has a radius of 114 feet, even larger than Hayesville, and handles perhaps a similar amount of anticipated traffic as would be in the center of Murphy.
I’ve driven on roundabouts in New England (and England), and they function well when correctly planned and with sufficient room. In Woburn, Mass., an easily traversed roundabout handles ample traffic, but with a radius of about 400 feet.
While the IDEA of a roundabout is appealing for its potential free flow of traffic, the needs and requirements of a roundabout are highly dependent on the
available radius of that circle to handle the volume of traffic. Even using our local roundabout examples, we simply don’t have the radius or the room at this major
intersection in Murphy.
The writer is a resident of Marble.