>>VO: Today our communities face a growing number of transportation challenges
As traffic volumes grow and congestion worsens
motorists, pedestrians and cyclist confront greater risks at intersections
we need balanced solutions that improve safety and keep people moving
at the Federal Highway Administration our first priority is safety
and our mission is to improve mobility for all users of our nations highways
In support of these goals we've been evaluating innovative intersection designs for many years
Where they've been built travelers save time, agencies save money
and communities benefit from safer facilities
One innovative intersection design, the Diverging Diamond Interchange
implements unimpeded, left turns onto the highway
by directing the traffic flow to cross over to the other side of the interchange
and then cross back before leaving the interchange
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>>Joseph Hummer: Alternative intersections are worth considering because the deliver more for less
They get more cars through the same spaces then in a conventional intersection
and they deliver better safety then a conventional intersection
>>VO: At many intersections the critical question is how to address left turns
How they are treated has an impact on the efficiency of the intersection
and can reduce a crash type that tends to be quite severe [music]
Missouri opened the first Diverging Diamond Interchange, DDI
in the United States in 2009
and several states have since adopted the DDI to ease traffic congestion at busy interchanges
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At first glance the DDI looks different than the traditional freeway interchange
with cross over locations at each end of the bridge. This may seem unusual
but these cross over points are actually the reason why the DDI functions so smoothly
They allow for better traffic flow
and simpler left turns between the interchange and the freeway
Since driving the DDI is a little different than what most of us are used to
lets take a closer look at how a DDI works
It's as simple as 1, 2, 3
1, a DDI uses the same basic diamond interchange layout
that is common in the United States. It consists of two entrance ramps and two exit ramps
2, all of the right turns in the DDI look and work the same way
as they would in a typical diamond interchange
whether turning right from the crossroad to the entrance ramp
or turning right from the exit ramp to the crossroad
drivers keep to the right side of the road or ramp
3, the cross over intersections are what make DDI's different than typical diamonds
they are actually very intuitive
The cross overs are designed to gently transition, or diverge traffic
from the right side of the road to the left side of the road
Drivers going straight cross over a second time
back to the right side of the roadway
Drivers turning left from an exit ramp or to entrance ramp
are already on the left side of the road
They no longer cross paths with other traffic
A major safety advantage that results in more efficient operations
Guide signs and pavement markings are used throughout the interchange to direct drivers. It's that simple
The Diverging Diamond Interchange boasts a number of safety improvements
It cuts the number of potential vehicle conflict points in half
>>Smith Siromaskul, P.E.: Not only are you decreasing the number of conflict points overall
you're also decreasing the speed at which those conflicts happen
Most of the accidents that you can have, most of the conflicts that you can have now
are sideswipe and that can be mitigated for with proper signal design
with making sure you have the right sight lines and things like that
>>VO: Good design principles, signage and pavement markings
make driving through a DDI simple and intuitive
>>Smith Siromaskul: So no longer are you going through an open space like in a diamond
where you can get hit from the right the left and the front
but everytime you go through an opening in a DDI you're only conflicting with one path at a time
and that makes a huge difference
>>VO: Although DDI's are usually deployed at locations with heavy traffic volumes
DDI's have proven to be beneficial to pedestrians and cylclists as well
Pedestrians can be accomodated either on the median in between the two cross overs or along the side of the road
>>Donald Saiko, P.E.: After the year that the first one was built we hired a consultant to look at the performance
and actually look through it, look at the crashes and compare those five years previous
to when we opened it up and we found that accidents were reduced by 53%
>>VO: A Missouri DOT survey showed that 87% of drivers
feel safer in the new Springfield DDI compared to the previous diamond interchange
>>Daniel Avila, P.E.: The commentary that I get when I interview people is, 'So how was it?' and they say, 'What?'"
I didn't even notice there was there was anything different
>>VO: Diverging Diamond Interchanges can usually be built on the existing bridge deck when retro fitted
For new interchanges fewer lanes are required
so there is less bridge structure and need for right of way
DDI's have been implemented in several states in a variety of roadway conditions
they can be built with the freeway over or under the crossroad
and at crossroads with as few as two lanes, or as many as ten
>>Bastian Schroeder, Ph.D.,P.E.: The intended use of a DDI is to accommodate those heavy left turns
>>VO: At the higher traffic volumes the DDI has less delays, fewer stops
reduced stop times, and shorter queue lengths compared to the conventional diamond interchanges
Delays were reduced by 15-60% and throughput was increased by 10-30%
With it's simple, unopposed left turns, the efficient two phase operations
narrower bridge structure width, lower costs
fewer conflict points and dramatically increased through put
The Diverging Diamond Interchange is a welcome edition to the nations roadways
The Federal Highway Administration encourages communites and agencies
to consider the Diverging Diamond interchange, to improve the efficiency and safety of existing interchanges
as well as and option for new interchanges. For more information about the Diverging Diamond Interchange
and other innovative intersection designs
please visit safety.fhwa.dot.gov
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