A radical new interchange design that sees drivers briefly cross to the ‘wrong’ side of the road has been found to both improve safety and save money.
The diverging diamond interchange (DDI) has been steadily gaining popularity in the US since 2009, when the first interchange of its kind opened in Missouri.
The design takes the form of a standard diamond interchange – but with a twist. From the highway, the interchange looks like any other diamond interchange form, with one ramp off the highway and one ramp on to the highway.
However, on the cross road there is something quite different. As traffic approaches the first ramp, the intersection is adjusted so that traffic will be placed on the opposite side of the road. At the second set of ramps there is another crossover intersection that returns traffic back to the right side of the road.
Both directions of traffic are diverted on a freeway overpass to the opposite side of the road, eliminating the need to cross the path of oncoming cars or wait for a green light to turn left (or right in Australia and New Zealand, for driving on the left side of the road).
A DDI offers fewer conflict points (14 for DDI, 26 for a conventional diamond solution), and these points are spread out throughout the interchange. The design also offers better sight distance at turns. Because the left turns are now simplified, there is often no need for extra lanes that are built just for the left turning traffic. The design also simplifies the traffic signals by eliminating the need for left turn signals.
US road agencies have embraced DDIs. Since 2009, 62 DDIs have popped up across 22 states. The US Federal Highway Administration (FHWA) describes the interchange design as: “An innovative, proven solution for improving safety and mobility at interchanges”.
The FHWA also lauds the savings a DDI can present, which in some cases can be as much as 75% less than an equivalent conventional diamond or single point urban interchange.
“DDIs often require fewer lanes than a conventional design to handle the same amount of traffic,” the FHWA writes on its website. “A smaller project footprint means less right-of-way is needed, resulting in fewer impacts to adjacent areas. All of this makes a DDI alternative cheaper, easier, and quicker to construct.”
New study proves benefits
A study published this year has found DDIs offer even more applications and greater safety than previously thought.
Safety Evaluation of Seven of the Earliest Diverging Diamond Interchanges Installed in the United States compared diverging diamond interchanges to the common classic diamond setups, which force exiting cars to make turns across traffic.
The team analyzed seven of the earliest DDIs in the United States. Four were in Missouri and other sites were in Kentucky, New York, and Tennessee. The team collected more than 28 site years of crash and other data before intersection conversion and more than 19 site years of data after their conversion.
Previous studies indicated that the DDI should only be considered in locations with a high traffic volume of left turning traffic, or if there was a strong directional movement during the peak hours.
This new study showed that the DDI can actually be applied in most situations when a diamond interchange form would be used with traffic signals. The study also demonstrates that the DDI becomes a more effective interchange option versus other diamond interchange forms when there is more traffic in general.
The report authors concluded: “Clearly, DDIs offer potential safety benefits, and agencies should consider them strongly as replacements for conventional diamonds.”
History of DDIs
American Gilbert Chlewicki is credited as the ‘father of the DDI’. While a grad student in 2000, Chlewicki wrote a term paper investigating the benefits of DDIs.
While on a trip to France, he was surprised to find his tour bus driving through a familiarly diamond-shaped intersection outside of Versailles. Although France has used DDIs since the 1970s, it was Chlewicki’s ‘rediscovery’ that triggered their newfound popularity in the US.
He then wrote and presented a ground-breaking paper on the DDI at a Transportation Research Board (TRB) sponsored conference in 2003. He went on to become the primary designer of the first DDI that was proposed in Maryland, although it was ultimately was not selected as the preferred alternative due to the low traffic volumes, which made any alternative viable.
DDIs locally
However, not everyone is convinced DDIs are an ideal solution. WA Main Roads is investigating the suitability of a DDI project, with help from the Curtin Monash Accident Research Centre (CMARC) and ARRB, who are testing whether the design will prove too confusing for Perth motorists.
The research comes after WA Main Roads commissioned a separate study into the feasibility of a DDI near Perth Airport, by Perth traffic design company Urbsol.
In this instance, Main Roads opted to go for an alternative single point interchange.
Urbsol director Nigel Matthew, who did traffic modelling for the $1 billion Perth Gateway project, told News Corp that diverging diamonds had the potential to work well, but that, “driver familiarity is always going to be a challenge”.
“What makes it unique is that vehicles cross over to the wrong side of the road ... which means there are some dangerous aspects to their design in terms of road safety,” he said.
“You have cars crossing to the wrong side of the road in front of each other and you usually have a concrete barrier running down the middle of these things, so there are head-on collision risks.”
The joint CMARC/ARRB study will use a $700,000 driving simulator to test whether Perth motorists will find the diverging diamond too difficult to negotiate.