Re: grade-separated roads (with ramps) vs roundabouts vs signalized

On May 3, 8:35 am, John Lansford <jlnsf...@xxxxxxxxxxxxx> wrote:
Larry Gross <gross.la...@xxxxxxxxx> wrote:
On May 2, 10:28 pm, John Lansford <jlnsf...@xxxxxxxxxxxxx> wrote:
Paul S Wolf <p...@xxxxxxxxxxxx> wrote:

On 5/2/2007 7:58 AM, Rothman wrote:
On May 2, 3:36 am, "Paul D. DeRocco" <pdero...@xxxxxxxxxxxxx> wrote:
"Steve A." <zoningper...@xxxxxxxxx> wrote

Roundabouts, in my opinion, are an upgrade
from four-way stop, an upgrade from most traffic signals on 2-lane
roads and light-volume 4-lane roads, and a downgrade otherwise.
My experience with Massachusetts rotaries, which are all one-lane, is that
they are better than lights up to a certain traffic level, above which they
bog horribly, and lights become a win.

To say all Massachusetts rotaries are all one-lane is somewhat
misleading.

Your experience with Massachusetts rotaries is immaterial.

Modern roundabouts are very distant cousins to Massachusetts Rotaries.
They're both round, and traffic goes around them in a counterclockwise
direction, but that's about the only thing they have in common.

The diameter of the central island of a Rotary like the two on 128 in
Gloucester is anywhere from 300 to 500 feet. A typical one lane modern
roundabout, would have an central island diameter of 120-150 feet.

Actually, the central island in a modern roundabout is considerably
less than 120' in diameter. That is closer to the outer diameter of
the circular roadway; the central island (usually grass and surrounded
by a truck apron) is closer to 50-80' in diameter and as large as
needed to gain the desired speed control for the roundabout itself.

hmmm... is there a functional difference .. in terms of moving traffic
between a roundabout, circle or rotary... or are they just different
names for very similiar functioning infrastructure?

No, a modern roundabout and a "rotary" are completely different
animals. The rotary was just a circular intersection, often many
hundreds of feet in diameter, that had roads coming off it like spokes
on a wheel. Speeds were higher and the biggest problem was that
traffic IN the rotary had to stop for traffic entering it.

Hold on there, John. You're talking about traffic circles, where
traffic IN the circle had to yield to incoming traffic. In New
England rotaries, incoming traffic yields to the circular traffic,
although the circles can be quite large.

.

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