The cable guide

Article July 12, 2007
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In the summer of 2004, Florida’s Turnpike Enterprise (FTE) embarked on an ambitious project to install median guardrail along 187 miles of highway to prevent crossover crashes and the resulting fatalities. This safety effort was prompted by an escalating crash history. In 1995, there were three fatalities recorded on Florida’s Turnpike. By the year 2004, this number had grown to 56, with FTE continuing to monitor the trend. As a result, FTE accelerated the already-planned funding of a safety program designed to redress the problem.

The $65 million project included Turnpike mainline sections in Miami-Dade, St. Lucie, Indian River, Okeechobee, Osceola, Orange, Sumter, Lake and Hillsborough counties.

When construction was completed in April 2005, the crash and fatality numbers dropped significantly. Since the installation of the median guardrail, FTE has averaged fewer than five median crossover crashes and five fatalities per year, according to state crash data.

 

Piloting the move

The median guardrail project provided an opportunity for FTE to explore a product not used previously in Florida—a high-tension steel-cabled barrier system. Florida’s Department of Transportation (FDOT) has not historically used cable barrier systems, non-tensioned or tensioned. Other states have utilized hundreds of miles of the three-rope system (non-tensioned) that have been in place for years.

FTE developed a pilot project as part of the Miami-Dade median protection project. Three cable barrier manufacturers—Brifen USA, Trinity/Cass and Safence—were invited to participate in the venture, which covered about seven miles of the Turnpike mainline. With the median protection projects, FTE’s general approach was to utilize double-faced guardrail. However, the cable barrier manufacturers asserted that their cable systems were less expensive to install and maintain, as well as faster to repair, than guardrail.

Some design considerations included defining proper dynamic deflection, establishing appropriate placement from the edge of travel lanes and determining the appropriate approach slopes. In determining the appropriate dynamic deflection, each manufacturer had a different deflection distance requirement to facilitate design. FTE chose to maintain one constant deflection requirement of 10 ft or below since this was a criterion that all three systems could meet.

In establishing the most appropriate placement of the wire rope, it is FDOT policy that roadside barriers for interstate facilities must be a minimum of 12 ft from the travel lane. Providing the 12-ft offset from the travel lane met the 10-ft deflection requirement.

In evaluating approach slopes, the minimum recommended approach slope for each system was 1:6. Given the 64-ft-wide median and flat longitudinal profiles, minimum grading was required. The only areas of special considerations were superelevated horizontal curves where minimum grading was required to meet the 1:6 approach slope criteria.

 

Tension situation

During construction, a number of issues arose, such as maintaining tension and preserving the material properties of each manufacturer’s wire rope. On the issue of maintaining tension, two of the systems were impacted by vehicular crashes while the project was still under construction, and it was determined that the wire rope did not maintain the appropriate tension after the systems were repaired.

Upon further investigation by the manufacturers, it was established that ambient air temperatures were insufficient for tension calibration, which meant the systems had to be tensioned based on material temperature.

In regards to preserving material properties of the wire rope, one manufacturer discovered that the specified resilient modulus for the wire ropes delivered did not meet their materials’ specifications. As a result, that manufacturer’s cable system had to be field-stretched in order to meet the specification.

Other issues included necessitating the placement of end terminals for the cable barrier systems. At the time of construction, one manufacturer did not have an approved end terminal per NCHRP and FHWA policy, which meant it had to be protected by existing guardrail at terminal locations.

Through extensive coordination with the manufacturers and construction personnel, all issues with installation and maintenance of wire ropes were resolved. Construction for the Miami-Dade median protection project, which included the cable barrier pilot project, was completed in August 2005.

 

Covering repairs

Due to limited experience with the cable barrier systems in the state of Florida, some of the maintenance issues that were encountered included training personnel in cable barrier repairs and monitoring tension in the wire ropes. This would be addressed in FTE’s cable barrier project for canal protection, which would follow a year later by including in the specification a requirement that, during construction, manufacturers are to train all maintenance personnel not only in the repair of each system, but also in the use of tension meters. In addition, during FTE’s cable barrier project for canal protection a cable barrier maintenance plan was developed to provide guidance for maintenance personnel.

Based on FTE repair data, the average time to repair the cable barrier systems now runs from 15 to 30 minutes. The average number of posts requiring repair is six per incident. It usually takes more time for the maintenance crews to get to the scene than it does to make the repairs.

It appears as though the cost savings that were anticipated are actually being realized. Initial construction costs for the cable barrier were about 75% of the cost for single-face W-beam guardrail per linear foot. The cost of maintaining the system was initially higher than the FTE staff had anticipated, similar to the maintenance costs of guardrail. Some of this cost was attributed to the learning curve needed by state maintenance forces to become familiar with the product. Those costs have now fallen below the cost to maintain guardrail.

Over the past 18 months, with more than 20 accidents and impacts, all vehicles have been captured or redirected by the median cable barrier system. No injuries have been reported, and there have been no crossovers within the pilot project limits. Lessons learned during design, construction and maintenance can now be applied to future cable barrier projects. FTE continues to refine the procedures for implementing the cable barrier systems.

 

Five-point star

With the success of the median guardrail project, FTE next turned to the issue of the hundreds of miles of canals that parallel the Turnpike. According to Cheryl Doherty, P.E., a PBS&J GEC project manager for the FTE and project manager for the canal protection program, the canals were created by the need for fill during the original construction of the Turnpike in the 1950s. Errant vehicles driving into these canals have resulted in numerous fatalities.

From 2001 to 2006, there were 32 fatalities on the Turnpike involving vehicles leaving the roadway and driving into the canals. Although these canals meet the state’s design requirements for offset from the roadway (60 ft from the edge of the travel way), vehicles have traveled up to 100 ft into the canals, resulting in fatalities.

The canal protection program became another opportunity to use the pre-stretched, high-tension, wire-rope cable barrier system.

The canal protection project consists of barrier devices in the form of either guardrail or cable barrier. The high-tensioned cable barrier systems provide flexibility to maneuver around existing elements such as trees, drainage features and utilities. As with the median barrier pilot project, the cost incentives associated with construction and maintenance greatly enhance the utilization of the cable barrier devices.

The initial canal protection project consists of 93 miles in Palm Beach, St. Lucie and Miami-Dade counties and calls for 54 miles of cable barrier and 39 miles of guardrail. Still under construction and with a programmed cost of $20 million, the project has an anticipated completion date of August 2007. Meanwhile, the remaining sections of the Turnpike mainline that are targeted for canal protection will be completed over the next 10 years. At a total cost of $61.5 million, this will provide canal protection along 259 miles of Florida’s Turnpike.

According to FTE Executive Director Jim Ely, the new canal protection system is an initiative that will close up all of the canals and deep retention ponds in the Turnpike System “to improve safety for our customers.”

Based on lessons learned in the design of the high-tension cable barrier system on the Turnpike, FTE developed a cable barrier design guide to direct future project designers. Information was developed not only from the experience gained in the median cable barrier pilot project, but also by coordinating with the FHWA Safety Office, incorporating material FTE roadway representatives brought back from Transportation Research Board meetings and the knowledge gained from work on the canal protection project.

For example, a lesson learned in the development of both the median cable barrier and canal protection plans is that high-tension cable barrier suppliers have differing line post spacings. FTE, in coordination with officials in Florida’s Department of Transportation Central Office, limits line post spacing to 16.5 ft. Some manufacturers supply high-tension cable barrier systems with line post spacing up to 20 and 30 ft.

The median barrier and canal protection safety improvements are part of the Turnpike’s Five-Point Mainline Safety Program. This initiative outlined an overhaul of the Turnpike mainline in the areas of crossover accident prevention and emergency response. Key components of the Five-Point Mainline Safety Program are the Turnpike’s barrier protection systems, public awareness, holiday safety breaks, expanded law enforcement presence and incident management.

Chief Jim Lee, former commander of Troop K, the Turnpike’s designated Florida Highway Patrol unit, recognized that completion of the barrier protection systems is already paying safety dividends. “There are a number of people still alive as a result of these guardrail installations,” he said.

The median barrier and canal protection projects, including the cable barrier pilot project, have provided tangible and rewarding results for the Turnpike.

To illustrate the effectiveness of the program, consider that recently a dump truck crossed the median, impacted the system and was redirected. The truck took out 56 posts but did not penetrate the barrier. Because the median cable barrier system was in place, a crossover crash was avoided and potential fatalities were avoided.

About the author: 
Sloup is the Turnpike design engineer for Florida’s Turnpike Enterprise. Cook is a consultant with PBS&J. Sloup and Cook are based in the Turnpike’s headquarters in Ocoee, Fla.
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