Straightening the S-Curve

An infamous and dangerous S-curve along Interstate 59 in Laurel, Miss. is now only a memory, replaced by a roadway successor destined to improve traffic flow and reduce accidents.

Newly constructed with an elevated bridge, the realigned interstate replaces the non-standard S-curve near downtown Laurel. No longer will motorists navigate tight radii, short acceleration and deceleration lanes, narrow bridges and shoulder-less roadways.

"There's dancing in the streets," former Mayor Susan Vincent told a crowd at the ribbon cutting in September. "The S-curve is straightened out, and now the 'S' stands for safe."

Originally constructed in 1961, the S-curve was engineered around a nearby hospital and public housing project. However, the three-quarter-mile stretch of highway required motorists to slow to 40 mph, and soon that portion of I-59 had reaped a history of traffic jams, tragic accidents and even fatalities. Desiring to eliminate the S-curve's long list of problematic and safety issues, the Mississippi Department of Transportation (MDOT) retained Garver in the early 1990s to design a route improvement.

Garver's services included roadway and bridge design, comprehensive construction plans and phasing, traffic signalization, overhead and understructure lighting and signage. Utilizing surveys provided by the state, Garver studied two alternative routes and designed a project that reconstructed I-59 along a new alignment, crossing six streets and a railroad track. The design included a 1,980-foot, six-lane, elevated bridge structure that featured horizontal reverse curves with greater radii. In addition, 1,350 feet of elevated ramps—two on-ramps and one off-ramp—connected to the elevated portion of the bridge.

After finishing the original design, the project was placed on hold pending the completion of the public housing relocation. During this period, the bridge construction industry realized a significant spike in structural steel prices. Given that the original design called for a structural steel superstructure, MDOT developed a preliminary cost estimate and concluded that as much as $10 million could be saved by replacing the structural steel members with prestressed concrete.

MDOT selected Garver in 2005 to redesign the structure using the more economical concrete option. Changing to concrete girders required a significant modification of the bridge span configuration to reduce the span lengths within the limits applicable for prestressed girder use.

"We asked Garver to redesign this interstate improvement project, which had been on the shelf nearly 20 years, in part because they produced the original design in the early '90s," said Steve Twedt, Mississippi Department of Transportation District Six engineer. "The redesign included changing the geometry to relocate a ramp, redesigning the superstructure from steel girder spans to pre-stressed concrete beams, and putting the plans into electronic format."

Garver designed the project on a fast-track schedule after the city of Laurel participated in MDOT's Highway Enhancement through Local Partnerships program. Because MDOT funds were not available for the project until 2008 or later, the city of Laurel accelerated the completion date by securing $32 million through bonds issued in October 2005. As part of the agreement, the Mississippi Transportation Commission then assumed the debt.

"It would have been years before the project could have been scheduled and let to contract," said Garver Project Manager Steve Haynes, PE. "However, because the project was supported by the local community, the project's construction moved forward, and we were able to respond on short notice and quickly provide services."

Garver submitted final plans for construction on schedule in March 2006, allowing MDOT to let work by June 2006.

Garver's design also included maintaining four lanes of traffic during construction as 25,000 vehicles use the interstate daily. By August 2008, the bridge's northbound lanes were opened and motorists began crossing the new bridge. The S-curve reconstruction project was completed in September 2009.

"We are pleased that the reconstruction on this section of highway is now completed. It will allow traffic to flow more efficiently and provide motorists with easier and safer commutes," said MDOT Southern District Transportation Commissioner Wayne Brown.

Interstate Widening

With remaining funds available after completion of the S-curve project, MDOT hired Garver to provide construction plans to widen and overlay an additional 2-mile stretch of I-59 immediately north of the S-curve reconstruction project. Five bridges will be widened, acceleration and deceleration lanes will be lengthened, the 4-foot median will be widened to 15 feet, 11 ramps will be adjusted to tie into the improved lanes and the existing roadway will be overlaid.

"I-59 through Laurel is being completely transformed," Haynes said. "While the overall footprint of this widening project along with the redesigned S-curve may not be large, their lasting benefits will impact all motorists traveling through the city. It really is fulfilling to be a part of these vital improvements."

Due to funding requirements, Garver produced and submitted the final construction plans in less than three months to accommodate the tight schedule. Helping keep the project on the fast track, the entire project will be constructed without the purchase of additional right-of-way. Construction began in October and will be complete in fall 2010.