Underpasses Explained

Urban underpasses are the key to central city congestion reduction given that there is no room to widen streets and there is no desire to build intrusive flyovers. Here is a short video that explains them.

Honolulu would greatly benefit from at least three express underpasses. One from Nimitz to Alakea and Halekawila, another on Kapiolani Boulevard at the Kapiolani/Date/Kamoku intersection by Iolani School, and a third by having a lane of Kalakaua Avenue go under Kapiolani Boulevard.

Here is a video of our traffic simulation that explains the one lane express underpass from Nimitz to Alakea and Halekauwila. Observe that the left turns that take a long time to receive green and spill over and block lanes on Nimitz Highway flow non-stop to both Alakea St. and Halekauwila St. Also the westbound (Ewa) flow has green light all the time except when there are pedestrian crossing requests.

The simulation screen-shots below show the before/after situation on Kapiolani Boulevard at its intersection with Date and Kamoku streets. The one lane per direction underpass visibly reduces congestion on Kapiolani Boulevard and allows for more green time to be allocated to the other sides of this complex intersection. This reduces their congestion and shortens the waiting time for the pedestrian crossings.


Because almost 50% of peak hour traffic on Kapiolani Boulevard and nearly 100% of its off peak traffic will use the underpass, the risk of collisions and pedestrian accidents also reduces substantially.

At least two of the existing surface lanes are maintained because these are "low clearance" underpasses which are easy to fit in a crowded urban setting. London, Paris, Seoul and Singapore have many such underpasses. Any vehicle such as a standard transit bus and lower will be able to use them. Taller vehicles (amounting to about 2% to 4% of traffic) must use the regular lanes. Also when the subject street has green, most of its traffic will opt to use the surface lanes.The underpasses will be equipped with sumps and pumps to remove storm waters.

Castle junction on the windward side of Oahu is an exception because for that location a flyover rather than an underpass would be shorter, safer and much cheaper to implement. The one lane flyover would replace the very busy Kaneohe-bound twin left turns that on some occasions cause queues to grow near the tunnels and block the lanes to Kailua. In the peak periods the intersection will improve from Level of Service F in AM and PM peaks now, to D in the morning and B in the afternoon peak periods.

More on our analyses of underpasses in these professional articles:

Dehnert, G. and P. D. Prevedouros, Underpasses at Urban Intersections: Investigation and Case Study. ITE Journal, Vol. 74, No. 3: 36-47, March 2004. Received 2005 Institute of Transportation Engineers Van Wagoner Award.

Prevedouros, P. D., J. K. Tokishi and K. Chongue. Simulation of Urban Underpasses for Traffic Congestion Relief. 10th International Conference on Applications of Advanced Technologies in Transportation, ASCE, Athens, May 2008.

Yu, A. and P. D. Prevedouros. Left Turn Prohibition and Partial Grade Separation for Signalized Intersections: Planning Level Assessment. ASCE Journal of Transportation Engineering. In review, 2012.