Holding down the fort

Transportation Management Article December 06, 2007
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The I-279 Fort Pitt Bridge and Tunnel in the city of Pittsburgh is a major link in the transportation system for Allegheny County and western Pennsylvania. I-279, known locally as the Parkway West, connects downtown Pittsburgh with major routes west and south including I-79, U.S. 22-30, PA 60 and Pittsburgh’s International Airport to areas north and east of the city, I-376 (Parkway East) and the Pennsylvania Turnpike.

As a result of Pittsburgh’s population shifts, demand for higher capacity on the region’s roadways was destined to increase. The Fort Pitt Bridge was designed to hold four lanes of traffic in each direction, and construction began in 1956 to replace its two-lane predecessor, the Point Bridge. Spanning the Monongahela River, the Fort Pitt Bridge is 1,217 ft in length. The bridge was opened to traffic in 1959—15 months before the opening of the 3,614-ft-long Fort Pitt Tunnel, which travels under Mount Washington leading to the South Hills of Pittsburgh.

Today, over 106,000 motorists utilize the tunnel and over 140,000 motorists use the Fort Pitt Bridge daily. A large percentage of motorists using these facilities commute to their place of work in downtown Pittsburgh. Magnificent skyscrapers, large bridges and sports facilities make the view when exiting the Fort Pitt Tunnel and entering the city one of the most poignant and captivating gateways in the world.

Because people who live north and east of the city may work south or west of Pittsburgh, rush hour is not typical: Along I-279 it now occurs in the morning and the evening in and out of the city. To say the least, the Fort Pitt Bridge and Tunnel have seen their share of traffic over the past four decades.

 

The traffic meeting place

After 40 years of wear and tear, the bridge, tunnel and Parkway East ramp (connecting I-279 to I-376) were in severe need of repair. But rehabilitating such heavily traveled routes would be no easy feat; studies to determine how to close the structures for rehabilitation and detour traffic began in the early 1980s. Just as the Monongahela River and the Allegheny River meet to form the Ohio River a few hundred feet west of the bridge, many ramps with high traffic volumes meet at the Fort Pitt Bridge. Closing the bridge and the ramps leading up to the bridge all at the same time was an option in the early stages of the project. However, the decision was made to perform various stages of the project separately in an effort to minimize the impact a total closure would have on the region.

Work on the tunnel facade was performed first with nighttime lane restrictions to minimize the effect on daily commuters. Work on the bridge piers was performed as a separate contract, and with this work occurring underneath the bridge, motorists were not affected. The ramps leading on and off the bridge were reconstructed in separate phases to minimize the overall effect to motorists. That left the final dramatic phase of the project: work on the main bridge span and tunnels.

 

Strengthening the roster

Detouring traffic off a main conduit into a busy city created its own challenge: The two main detour routes that would be used during the closure of the Fort Pitt Bridge and Tunnel already carried large volumes of daily traffic. The Liberty Bridge handles more than 63,000 vehicles per day, and the West End Bridge sees 32,000 vehicles on an average day. Motorists who use these bridges experience delays during both the morning and evening peak periods due to the heavy traffic volumes in and out of the city. Placing detoured traffic onto these detour routes—without any mitigation to those detour routes—would have caused gridlock throughout the day.

Actual mitigation work on the detour routes began many years before the closing of the main span of the bridge and the tunnel. Roads and bridges to be utilized as detour routes were reconstructed and rehabilitated as early as 1990, the year reconstruction of the West End Bridge began. Some of these projects included the refurbishing of the Smithfield Street Bridge, a historical landmark that has transported Pittsburgh travelers since the 1880s.

The Boulevard of the Allies, which runs high above I-376 on the bluff overlooking Pittsburgh’s south side, was badly deteriorated. This roadway was reconstructed in two separate phases, the last phase being completed just three months before the March 2002 closing of the Fort Pitt Bridge and Tunnel.

The deck of the Liberty Bridge was milled and resurfaced to provide detoured traffic with a ride over that span. The West End Bypass and Rte. 51 from the West End Bridge to the Liberty Tunnel were rehabilitated. The Liberty Tunnel roadway was reconstructed, and a new Liberty Interchange at the south end of the Liberty Tunnel was designed and built to help ease congestion from the South Hills and to help expedite traffic during the closure of the Fort Pitt Bridge and Tunnel.

 

Reversing and re-entering

The West End Bridge has traffic signals on both the north approach and the southern West End Circle. Similarly, the Liberty Bridge and Tunnels have four sets of traffic signals from the merge with I-579, the Crosstown Expressway and the South Hills side of the tunnel. To facilitate the movement of 70,000 vehicles a day, PennDOT District 11 implemented a strategy of diverting this traffic along two main corridors and making these two detour routes free-flow. The district’s strategy was to divert traffic from southbound I-279 and the northern portion of the city’s Golden Triangle to the West End Bridge. Traffic from east of the city traveling along I-376 was diverted to the Liberty Bridge via the Boulevard of the Allies.

Traffic signals along both of these detour routes were disconnected to allow motorists continuous movement along the detour. When the traffic signals were turned off along these two detour routes, movements that normally could be made with the aid of the signal were eliminated. These movements required 14 additional secondary detours. In order to get motorists acquainted with the detours prior to closing the outbound Fort Pitt Bridge and Tunnel, the contractor was instructed to implement the West End Bridge and Liberty Bridge and Tunnel’s free-flow condition one week prior to the actual closing of the Fort Pitt Bridge and Tunnel.

In an effort to ease afternoon rush-hour traffic leaving the city of Pittsburgh, the district expanded the hours of the Liberty Bridge lane reversal system from the normal 3:30 p.m. to 6 p.m. three-lane outbound/one-lane inbound traffic pattern on the bridge to 2 p.m. to 7 p.m.

Because both the West End detour route and the Liberty Bridge/Tunnel detour route would have to merge into a single lane prior to re-entering southbound I-279 on the southwestern side of the Fort Pitt Tunnel, the department opened a hole through the existing concrete barrier normally separating traffic coming out of the tunnel from the Rte. 51 north and south ramps. By doing this, traffic following from the West End detour route could re-enter the Parkway West in a free-flow lane, while traffic following the Liberty route could re-enter from a single lane without either detour route having to merge into a single lane. This strategy proved extremely beneficial to motorists.

One unique strategy involved the complete change in the detouring of traffic during the 2003 construction season. The bid documents for the 2003 inbound closure of the Fort Pitt Bridge and Tunnel required the inbound traffic to be detoured via the West End and Liberty Bridges. However, the district, using what was learned from public reaction to the 2002 outbound closure and detours, decided to implement the same detours in 2003 for outbound traffic and thereby allow inbound traffic to use the recently completed tunnel and bridge deck.

Three inbound movements were created, allowing motorists to access the city of Pittsburgh and I-376 by going inbound through the outbound tunnel. Northbound I-279 traffic was the only inbound movement that actually had to follow a detour via the West End Bridge. By using this unique strategy, inbound traffic actually flowed with no detours in place since the northbound I-279 weave that normally occurs under the original operating conditions was eliminated. Morning traffic inbound on I-279, which typically backs up to the top of Greentree Hill, a two-mile delay, never backed up for the five months that the inbound traffic was redirected through the outbound tunnel.

As mentioned, eastbound I-376 traffic also traveled inbound through the outbound tunnel and traveled on westbound I-376 (again on what would be considered the wrong side) separated from traffic by a concrete median barrier. The challenge became how to get eastbound I-376 traffic back to its correct side. Concrete piers that carried Fort Pitt Boulevard between I-376 in both directions presented a major obstacle.

 

The squeeze play

The strategy implemented by the district squeezed eastbound I-376 traffic through two piers. In order to accomplish this safely, a 15-mph speed limit was enforced (possibly the only 15-mph speed limit ever instituted on an interstate highway in the U.S.). Because the opening was very narrow, all trucks over 40 ft in length were prohibited from using eastbound I-376 and were thus forced to follow a detour.

While the concrete barrier did have its share of scuff marks on it from motorists who disobeyed the 15-mph speed limit (or the occasional truck that did not follow the detour signs, did not fit through the opening and had to be pulled out), this strategy was very effective, maintaining flow for the motorists who used this roadway.

The district was able to monitor traffic conditions along the detour routes from the Pittsburgh Regional Traffic Management Center and provide motorists with real-time traffic messages via variable message signs and highway advisory radio along I-279 and I-376. The district also utilized tow trucks along these routes if motorists encountered disabled vehicles or if accidents occurred.

About the author: 
Cippel is an assistant district traffic safety engineer for PennDOT.
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