One of the best things that an engineering design firm and a construction contractor can do is to find a way to reduce the cost of an interchange or intersection using Innovative Design Concepts. These types of design concepts are widely understood in most transportation communities but may be new to many construction firms and construction executives.
In July 2010, the Federal Highway Administration (FHWA) published a 340-page document entitled, “Alternative Interchange Design Alternatives” – now considered the “bible” by transportation firms. It is widely used when firms are in the process of determining the best way to increase capacity while charging the least, especially when a government agency is involved.
This FHWA document is a helpful tool for both engineering firms and design and road construction contractors alike. If we, as an industry, are able to reduce the cost of an interchange while creating similar or more capacity, we can solve many problems for clients, elected officials, and the Local Department of Transportation. The types of designs that are most frequently utilized are as follows:
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- Traffic Signalization – we are all familiar with this type of intersection control for three, four, or five-legged intersections. The issue is capacity, delay and rear end accidents.
- Displaced Left Turn Intersection – this type of design reduces the typical 3-way signal from three signal phases to only two phases thereby increasing capacity and reducing delays.
- Median U-turn Intersection – the left turn movement at signalized intersections is a capacity killer, and this type of design moves the left turn to become a right turn.
- Restricted Crossing U-Turn Intersection – When a median opening is left unrestricted, and vehicles are allowed to wait in the median, a number of bad things can occur. This type of design requires a right turn, and then a U-turn at the next opening, resulting in a safer movement.
- Quadrant Roadway Intersection – when right of way is available, this type of design, can also eliminate direct left turns, or when there is not a sufficiently wide enough median for a left turn lane.
- Roundabouts – likely the best safety and operational solution to an unsignalized or even signalized intersection. Low speed operation and safe for all users.
- DDI or Diverging Diamond Interchanges – a way to vastly and safely increase the capacity of a typical diamond interchange or as an alternate to building an economical new interchange.
The above lists some of the more common alternative designs that are utilized to improve operations of intersections and interchanges.
As noted in the technical documentation page from FHWA, today’s transportation professionals are challenged to meet the mobility needs of an increasing population with limited resources. At many or most highway junctions or interchanges, congestion continues to worsen and there is nothing in the foreseeable future to reduce the traffic volumes or the congestion. Drivers, pedestrians, and bicyclists experience longer delays and greater exposure to risk. It is no secret, pedestrian accidents are increasing at an alarming rate and it is believed that it is largely due to the higher number of SUVs versus typical sedan-type vehicles.
Conventional intersections/interchange designs are often found to be insufficient to mitigate transportation problems. As a result, traffic engineers are investigating and implementing innovative treatments in an attempt to think outside the box, in order to save lives, with greater regularity.