Civil engineers in Missouri have published a series of papers on the safety of Diverging Diamond Interchanges (DDIs), which conclude they could cut the frequency of car accidents by half.
Seen in the US, a DDI diverts traffic exiting major roads by – assuming they are driving on the right as in the US – moving traffic onto an overpass or underpass that runs on the left side of the road. It aims to improve safety by removing the need for left turns that are found in normal diamond interchanges. Aside from a handful built in France in the 1970s, the system is relatively new, with the first built in the US in 2009.
Diverging diamond interchange
Source: Carlos Sun
Academics from the University of Missouri Civil and Environmental Engineering Department have published three papers looking at the safety of DDIs. These examined data collected over several years, taking into account variables such as traffic levels and local conditions. More than 10,000 crash safety reports were analysed. They looked at interchange safety before and after the introduction of a DDI.
The team found that fatal and injury crashes decreased by up to 73.3% after changing to a DDI, and overall crash frequency decreased by as much as 54%. The evaluation of the seven earliest DDI interchanges illustrated that a DDI should reduce crashes by an average of 33%.
Professor Carlos Sun co-authored two of the papers. He said: “Before our studies were released, there really was little information about the safety effects of diverging diamonds. This was leading safety research in terms of diverging diamond interchanges.”
The third paper published was by Henry Brown, a civil research engineer at Missouri. His team looked at practices for diverting traffic while constructing and later maintaining interchanges such as DDIs. Brown and the team surveyed project managers on construction and maintenance projects for a variety of interchanges to see which practices tended to work the best and the examined over construction plans to establish a set of best practices for the various interchanges.
He said: “There’s very little guidance out there on how workers can safely divert drivers and change traffic patterns while implementing DDIs, roundabouts and other designs.
“If workers are retrofitting a diamond interchange, they’re actually completely changing the traffic pattern and that just can’t happen by flipping a switch; it takes careful planning.”
They suggested that interchanges should be built in phases and then briefly closed to install signage to change traffic flow.
Pictured is the Aecom designed Pleasant Hill Road DDI, Gwinnett County, US.