Thursday, July 1, 2010

Diverging diamond interchange coming to Ashford Dunwoody & I-285

Diverging diamond interchange

AJC reports - A new interchange at Ashford-Dunwoody Road and I-285 could be under construction this fall, thanks in part to a new infrastructure bank run by the state.  State officials said Ashford-Dunwoody would be one of the first "diverging diamond" interchanges in the U.S., designed so that cars that pass over the bridge will switch to the left-hand side of the road, to streamline all the turning in the area.

A diverging diamond interchange is a rare form of diamond interchange in which the two directions of traffic on the non-freeway road cross to the opposite side on both sides of the bridge at the freeway. It is unusual in that it requires traffic on the freeway overpass (or underpass) to briefly drive on the opposite side of the road from what they are accustomed.

Like the continuous flow intersection, the diverging diamond interchange allows for two-phase operation at all signalized intersections within the interchange. This is a significant improvement in safety, since no left turns must clear opposing traffic and all movements are discrete, with most controlled by traffic signals.[1] Additionally, the design can improve the efficiency of an interchange, as the lost time for various phases in the cycle can be redistributed as green time; there are only two clearance intervals (the time for traffic signals to change from green to yellow to red) instead of the six or more found in other interchange designs. Some of the intersections in the design can be unsignalized. The left turn from the freeway off-ramp, for example, can form an auxiliary lane that then becomes an exit-only lane for the entrance ramp to the freeway in the opposite direction. Omitting the traffic signals for the left turn movements off the freeway only works well with single left turns and when short queues exist within the interchange on the arterial street.


Mamacita said...

Your sample states Interstate 75 and not 285. I believe that this will incur more wrong way traffic since it is confusing for all drivers. The amount of traffic will not diminish and the back up of autos in both directions on Ashford Dunwoody. One of the main problems there are cars cutting across lanes to get onto the entrance ramp for the freeway. If you simply put up a divider it would reduce problems.

Anonymous said...

That sample diagram appears at is not a Georgia DOT or Dunwoody diagram, which is why the lower road is not labeled "I-285").

What's not clear is how well these diamonds work when the traffic on the lower road (i.e. I-285 in our case) is so backed up that its entrance ramps are blocked. This is a large part of what causes the current congestion on Ashford-Dunwoody.

I'd hate for us to spend the significant sums of money and put up with construction barriers headache there (yet again) but only to find out that the Ashford-Dunwoody congrestion no better.

John Heneghan said...

Dunwoody Crier Article

Innovative re-design above I-285

Drivers will have to get used to driving on what seems the wrong side of the road, but the newly approved re-design of the Ashford-Dunwoody and I-285 interchange promises to ease congestion and improve traffic flow.

The Perimeter Community Improvement District has been awarded an $800,000 grant and a $684,000 loan from the Georgia Transportation Infrastructure Bank to begin work on an innovative re-design of the congested intersection. The design will use a “diverging diamond” interchange concept, the second in the nation, to add additional capacity without adding new lanes.

The redesign will re-stripe lanes, alter signal timing and eliminate most left turn lanes.

Yvonne Williams, PCID President said the diverging diamond design provides an interim solution.

“We were able to find a way to innovatively provide traffic relief while developing a beautiful entry way to the Perimeter-Dunwoody area,” she said. Williams adds that it is expected the interchange will eventually have to be reconstructed at an estimated cost of $170 million, but in the meantime, this project, projected at $8 million will help ease congestion within a year.

“If we can line up all the money, we hope to be under construction in 2011,” said Williams.

The PCID was awarded the grant by the State Road and Tollway Authority in a competitive process. SRTA board member Wendy Butler said the Ashford-Dunwoody interchange redesign stood out for a number of reasons: the bridge is geographically significant in terms of congestion and as an entrance to a major employment center and the redesign was both innovative and cost-effective.

“We found the redesign would have significant benefits with a minimum of capital expenditure. It should significantly improve mobility, relieve congestion and make the interchange safer,” added Butler.

Georgia Road Geek said...

From my blog, I have a link to a YouTube video of the diverging diamond in Missouri...

John Heneghan said...

Fox5 News Coverage