If you’re over on the highways and bridges side of the world diverging diamond design (DDD) may be known to you, at least by repute; if you’re on the goods-moving side, here’s an opportunity to learn about one of the more intriguing interchange design solutions out there.
The design is innovative and growing in implementation – there are a double handful current in the country and they are present in nineteen states – in that it replaces a standard diamond interchange by switching traffic to the opposite side of the road before traveling over an overpass or under an underpass, eliminating the need for left-turn lanes and for left-turning traffic to cross against oncoming traffic. Only two 2-phase traffic signals are required.
Eliminating the left-turn lanes and phases saves on both the width of the structure and lost time at the traffic signal. Traffic flows more freely in the direction it wants to go and fewer conflict points translate into greater safety.
One of the tremendous benefits of DDI is that you often add capacity without having to widen or replace the bridge. On one of our DDIs we were actually able to remove a lane on the bridge and add a pedestrian crosswalk while still increasing capacity.
This DDI interchange configuration has been evaluated by various agencies since 2003, and has been the subject of intensive study and simulation testing by the FHWA, which has verified that it is more efficient than traditional diamond interchanges when there is a high percentage of turning traffic. The agency also verified that the pattern was easy to understand and to negotiate by drivers.
At a time of many demands upon the city, state, and federal budget, DDI brings reduced costs, often furthering capacity increases while retaining and reusing the existing structure.
We hear all the time of innovation in project finance, project delivery, public partner participation and the like, but it is often lost in the shuffle that design innovation also brings enormous benefits to clients, opening a door out of the funding struggles and legislative inaction current in the industry right now.
Transportation consultant and design professionals are at the center of this transportation solution, and we are right in the bull’s-eye of that center, as it is one of the specialized areas of expertise we offer. If the parameters line up – the nature of existing traffic flow, the status of the current configuration, the necessary capacity to add – then diverging diamond designs aren’t diamonds in the rough at all, but sparkling jewels for the municipal client to consider. – lsm