Sagamore Rotary

Rotaries Versus Modern Roundabouts

Modern roundabout are superficially similar to rotaries and their important differences are not immediately obvious. As modern roundabouts are new to the USA few have experienced their benefits. With their bad experience of traffic circles it is easy to understand why engineers, decision makers, public agencies and the general public alike find it difficult to understand that a smaller modern roundabout operates very differently to Rotaries and that they have higher capacity and lower crash rates. However, many communities in the U.S. have now implemented roundabouts with great success.

The Sagamore Rotary (link to full report) was designed in the 1940’s. Many Rotaries were built in this time frame on the east coast. Rotaries worked well with low traffic volumes, but congestion, crashes and driver discomfort increased sharply at high traffic volumes. The inherent design defects to Rotaries are their large diameter, high speed entries, high speed weaving and lack of consistent signing and good pavement markings that produce an inconsistent and confusing message to drivers (lack of positive driver guidance). The Sagamore Rotary exemplifies the above problems.

Modern roundabout design is based on extensive research and engineering experience derived over the last 35 years primarily in the UK where very dense traffic conditions are prevalent. Properly designed high capacity roundabouts have proven themselves to perform extremely well throughout the US. There are good examples as widespread as California, Colorado, Florida, Michigan, Maryland, New York, Utah and many others. Below is a photo from the State of NY DOT of a recently opened modern roundabout that replaces an old rotary. Below is a photo of a recently opened modern roundabout in NY State that replaces an old rotary. The large diameter rotary shown in the photo above was performing very poorly (like the Sagamore rotary) whereas the new ~200’ diameter modern roundabout is providing very safe and efficient traffic movement for approximately two years now.
Despite their considerable success many in the US remain unconvinced about the benefits of roundabouts. However, as the above list indicates, they offer significant advantages over more conventional intersections when used in appropriate situations and when they are properly designed.

One main advantage of modern roundabouts is their considerable geometric flexibility. This enables the efficient connection of difficult roadway alignments. This geometric flexibility is possible due to the slow approach, entering and exit speeds inherent in properly designed modern roundabouts. Conventional signalized intersections do not have such geometric flexibility as higher vehicles speed requires larger centerline radii.

Although modern roundabout are relatively new to the US (about 300 have been built in the last 7 years) they are rapidly gaining acceptance. Modern roundabouts are a new ‘intersection control device’ that are worth considering as an alternative to conventional signalized intersection control. Alternative choice depends on the merits of each specific case. Modern Roundabout interchanges are becoming increasingly popular.