kim woods

Term Paper

Diverging Diamond Interchanges

Table of Contents

Abstract Page 3

Introduction Page 3

Current Interchanges Page 3

Diverging Diamond Interchange Page 4

Traffic Studies Page 5

Future Development Page 6

Conclusion Page 6

References Page 7

Abstract

Introduction

Traffic, we all have experienced the headaches it brings. At an intersection is where the potential conflict is most likely to occur, especially between two major traffic arteries. The goal of traffic engineering is to provide the most efficient means of alleviating the traffic conflicts. However, in areas with increasing traffic demands that the traditional methods have been insufficient in dealing with the higher traffic. Therefore a new intersection design was developed known as the diverging diamond interchange. The diverging diamond interchange utilizes channelization and a crisscross lane pattern to reduce traffic conflict points. Studies have shown that commuters using a diverging diamond interchange feel safer than they do using traditional interchanges. Along with being safer, the diverging diamond interchange is more cost effective than alternative interchanges by utilizing existing infrastructure for its design.

Current Interchanges

Interchanges are a specific classification of intersections which are constructed to provide un-interrupted flow for crossing traffic by changing the elevation between the roadways. Due to this, ramps are required to connect the crossing traffic and allow turning from one to the other. In the usual interchange traffic wanting to turn left has to cross oncoming traffic to enter or exit a ramp perpendicular to vehicle’s current direction. Therefore a left turning vehicle is at a greater risk getting involved in a serious collision when crossing oncoming traffic that may result in fatalities then the through or right turning traffic. Majority of the fatal crashes at an interchange occur from the unprotected left turn.

There are measures of left turn protection with minimal altercations to the design of the intersection. Designing an intersection with left turn protection adds additional cycles to the traffic signal providing more green time for left turning traffic and prevents oncoming traffic from moving. Also, when necessary, extra left turn lanes may be required storage isolating the through traffic. However, a longer cycle length due to left turn protection causes its own problems, in high traffic interchanges. This is because there is longer red time for traffic in other directions when the left turn cycles are added to at an intersection.

Furthermore, when there is too great of a demand for any particular interchange another issue arises, traffic can become backed up causing a delay even after adjusting the signal timing. Traffic backed up at one intersection far enough will affect the traffic flow at other intersections along the same traffic artery even when there are multiple lanes. To accommodate an increase in traffic volumes addition infrastructure is added such as super elevated ramps and bridges which can become costly depending on the layout and footprint of the new infrastructure. Is there a better way? To increase efficiency, the diverging diamond interchange may be the solution in areas where the current interchange design is failing to keep up with the higher traffic volumes.

Diverging Diamond Interchange

The Diverging Diamond Interchange or DDI features a crisscross flow direction with shallower angles at the intersections this configuration can be seen in the figure 1. Another feature in the design is the unique left turn, since the directions of travel switched the vehicles in the farthest left lane can perform their left turn onto the ramp in a similar manor as a right turn would be made at standard interchange. Therefore, no left turning ever crosses oncoming traffic while making the turn which is much safer and more efficient.

Figure 1

(Source from: Diverging Diamond Interchange Informational Guide by Schroeder, 2014)

The DDI design is relatively new to the United States the prior to 2000 this interchange design was unheard of until Gilbert Chlewicki, currently the ATS/American’s division director published his paper on this design titled “New Interchange and Intersection Designs: The Synchronized Split-Phasing Intersection and the Diverging Diamond Interchange” in July of 2003. The diverging diamond Interchange design caught Joe Bared, PhD, PE from the Federal Highway Administration (FHWA), attention. Bared, performed an evaluation of the design and released his results in 2005 with promising remarks for the design (Chlewicki, 2014). The design was untested in reality until 2009 where in Springfield, Missouri the first diverging diamond interchange was constructed. The 3 million dollar project between I-44 and the Missouri SR 13 cost significantly less than the traditional interchange of 10 million dollars. This was achieved because there was no substantial construction needed and the existing bridge over I-44 was adequate enough for the DDI. After opening, the response from the public was better than anticipated many and the theory behind the diverging diamond interchange of being efficient and safer was proven to be true (Chlewicki, 2014).

Traffic Studies

Safety is always a major concern for a new design, the DDI was no exception. A DDI in theory is safer than the standard diamond interchange because there is a reduction of points of conflict shown in Table 1. Majority of fatal crashes at traditional diamond interchanges occur when crossing traffic depending on the layout of the interchange there are 10 crossing points of conflict in the conventional diamond interchange. These conflict points generate the T-bone and head on collisions which often in fatal crashes. The DDI has only 2 points for crossing conflicts also the crossing itself has been modified to provide angles that reduce the severity of a crossing collision and discourages driving into oncoming traffic as well since the flow directions swap (Federal, 2016).

(Sourced from Crossover-Based Intersections by the Federal Highway Administration, 2016)

Before the first DDI was constructed in Missouri, the Missouri Department of Transportation (MoDOT) and the FHWA developed a traffic simulator to fully test the effectiveness and efficiency of the diverging diamond interchange design as well as optimize the location for signals and signs. During their study, the FHWA selected 74 participants to add a human factor to the DDI simulation the results were as follows in Table 2.

(Sourced from Drivers' Evaluation of the Diverging Diamond Interchange by Bared, 2007)

The results of the study shows that the DDI is viable alternative interchange design combining operational benefits with increased safety.

In Springfield, Missouri between I-44 and the Missouri SR 13 where the first diverging diamond interchange was constructed. Saw significant reductions in traffic congesting and in traffic collisions over the previous interchange after completion of the DDI. Post construction a public survey conducted by MoDOT found that 95 percent of drivers agreed the roadway was less congested. Another study shows that the total crashes declined 46 percent when comparing the average across the 5-year baseline to the one-year post-construction period of 24 percent reduction in total crashes (Shaw & Doctor, 2015). The DDI possible could become the preferred interchange design.

Future Development

The second diverging diamond interchange in Ohio began construction on September 28, 2016 at I-475 and Ohio SR 25 in Perrysburg. At the current interchange there has been major congestion the Toledo Metropolitan Area Council of Governments and the Ohio Department of Transportation anticipate that the DDI should help ease traffic movement through the congested SR 25 and I-475 interchange. The project costs 8.75 million dollars and should take a year to complete. The DDI will utilize the existing bridge over I-475 therefore reducing costs however the existing on and off ramps will be replaced to accommodate the DDI’s distinctive arcing approach and storage lanes. DGL Consulting Engineers designed the DDI and the project was awarded to E.S. Wagner as the general contractor (Maiolo, 2016).

Conclusion

References

Bared, J. G. (2007, September 25). Drivers' Evaluation of the Diverging Diamond Interchange (United States, Federal Highway Administration, Office of Safety Research Development and Technology). Retrieved November 2, 2016, from https://www.fhwa.dot.gov/publications/research/safety/07048/

Chlewicki, G. (2014). The Diverging Diamond Interchange Website. Retrieved October 31, 2016, from http://divergingdiamond.com/index.html

Federal Highway Administration. (2016, May 26). Crossover-Based Intersections. Retrieved November 05, 2016, from http://safety.fhwa.dot.gov/intersection/innovative/crossover/

Garber, N. J., & Hoel, L. A. (2015). Traffic and Highway Engineering (5th ed.). Stamford, CT: Cengage Learning.

Maiolo, J. (2016, October 5). ODOT Begins Work on Divergent Diamond Interchange. Perrysburg Messenger Journal, p. 1.

Schroeder, B. J., Cunningham, C., Ray, B., Daleiden, A., Jenior, P., Knudsen, J., & Kittelson & Associates, Inc. (2014). Diverging Diamond Interchange Informational Guide [PDF].

Shaw, J., & Doctor, M. (2015, December 1). Interstate 44 and Missouri Route 13, Springfield, MO; Diverging Diamond Interchange. Retrieved November 05, 2016, from http://safety.fhwa.dot.gov/intersection/innovative/crossover/case_studies/mo/