When you get down to it, roadside safety is about minimizing negative impacts––in some cases, quite literally.
It is about reducing the probability for harm by introducing positive factors into the equation. For example, effective planning, improved technologies and behavioral modification all play key roles in maximizing roadside safety.
Safety in work zones is no different. The biggest threat to work-zone safety occurs when motorists and workers come into conflict; when a car strays into a work-zone area, for example, putting all involved at risk. As part of 2009’s National Work Zone Awareness Week, then-U.S. House Transportation and Infrastructure Committee Chair James Oberstar pointed out this problem in his address on the floor of the House of Representatives. The issue is particularly relevant, Oberstar pointed out, in light of the need to reconstruct the U.S. transportation system.
“As a result of the nation’s aging highway infrastructure, the country is faced with unprecedented levels of reconstruction and maintenance projects,” Oberstar said. “And these levels will only increase in the near future.”
Financial barrier
Even before Oberstar’s call to action, the Federal Highway Administration (FHWA) had published 23 CFR Part 630 Subpart K – Temporary Traffic Control Devices Final Rule in 2007. The rule’s objective: to “decrease the likelihood of highway work-zone fatalities and injuries to workers and road users.” The new rule established requirements related to five items for federal-aid projects. States, it said, should:
Provide guidelines for determining when to use positive protection in work zones;
Use measures to minimize worker-
motorist conflicts;
Implement other traffic-control strategies (e.g., law enforcement) to minimize work-zone crashes;
Ensure safe entry/exit points for work vehicles onto/from travel lanes; and
Provide funding to ensure availability for these provisions.
The FHWA does not mandate specific requirements for introducing positive protection into a work zone. Instead, states are responsible for developing guidelines for determining in what situations and to what extent positive protection should be used.
The safest solution, of course, is to always use positive protection to separate motorists and workers while work is ongoing. But that is not realistic in today’s economic environment. Cash-strapped state departments of transportation (DOTs) have to prioritize where and when to spend their dollars to maximize the public-benefit return on the taxpayer’s investment in the nation’s infrastructure. Optimizing how resources are spent on state transportation networks is the No. 1 challenge today for DOTs nationwide.
When to be positive
For years the Texas Department of Transportation (TxDOT) had procedures in place for assessing the need for positive protection, but they had some drawbacks.
TxDOT decided it needed more comprehensive guidelines for when to introduce barriers into work zones. Better guidelines mean safer roads for workers and motorists alike, as well as a more cost-effective use of limited resources. So, at TxDOT’s direction, a team led by Jerry Ullman, manager of the Texas Transportation Institute’s (TTI) Work Zone and Dynamic Message Sign Program, conducted Project 0-6163: Improved Positive Protection Guidance for Work Zones. They came up with specific recommendations for when to introduce positive protection into work zones.
The reality is that every vehicle traveling through a work zone carries with it a probability of straying into the work-zone area. By looking at the number of vehicles traveling in a corridor, the probability that motorist-worker conflict would occur can be estimated. Using that data, Ullman and his team compared the cost of installing and maintaining traffic barriers with the societal costs associated with injuries and deaths resulting from work-zone-area intrusion crashes. From there, they came up with specific break-even recommendations for when Texas should introduce positive protection into work zones. Specifically, the objectives of the study were to:
Analyze the benefits and costs of using portable concrete barriers (PCBs) for positive protection in work zones;
Analyze the benefits and costs associated with moveable- and portable-barrier technologies that can be more quickly deployed and removed at work sites than traditional PCBs;
Analyze the benefits and costs of nonpositive protection devices to improve safety and reduce work-zone intrusion events in work zones; and
Develop implementation guidelines for these various technologies.
“Essentially we answered the question, ‘When does it become cost effective for Texas to use protective barriers in work zones?’” said TTI Assistant Transportation Researcher Vichika Iragavarapu.
Texas tweaking
TTI researchers used the National Cooperative Highway Research Program’s Roadside Safety Analysis Program (RSAP) to crunch their numbers. RSAP was developed to analyze the cost effectiveness of proposed roadside safety devices. Using RSAP data, traffic engineers can then determine the relative performance levels of the devices. RSAP was developed to determine permanent roadside devices, however, making it of limited use (in its original form) for evaluating positive protective barriers for work zones.
Ullman and his team modified RSAP inputs to model how PCBs might be implemented in a work zone. They obtained PCB costs pertaining to Texas, then used recent comprehensive crash costs (summarized in Table 1) and adjusted the RSAP encroachment rate to better represent work-zone situations based on other studies of work-zone crash frequency and severity. Researchers then modeled various combinations of work-zone configurations, roadway geometrics and traffic characteristics to evaluate the relative benefits and costs of protecting work spaces with portable barriers.
“We adapted program inputs and outputs to meet our needs,” Ullman said. “Ultimately what we used RSAP for was to determine a vehicle’s probability of hitting a worker or piece of equipment in a work-zone area given various factors, such as average daily traffic [ADT] and vehicle speed through the work zone.”
Once that factor was determined, researchers used crash-cost estimates from FHWA (updated to 2008 dollars) and RSAP to determine the break-even point of installing positive protection devices.
Three work-zone scenarios were of primary interest to the researchers:
Where no drop-off concerns are present and where positive protection is solely intended to prevent intrusion crashes with workers, equipment and work materials in the work space;
Where a drop-off exists some lateral distance from the travel lanes, and the work space is located between the travel lanes and the drop-off; and
Where a drop-off exists next to or a short lateral distance from the travel lanes, and the work space is located beyond the drop-off.
The closer, the safer
For the 70-mph divided highway scenario (see Figure 2), intrusion crash-cost savings alone can justify PCB protection, assuming ADT of at least 40,000 vehicles per day (vpd) with constant hazards in the work area (e.g., equipment and materials left within the work area even when workers are not present). These results can be extrapolated to look at other work-zone durations: for a six-month work zone, an ADT of 80,000 vpd is required for breakeven; for a two-year work zone, an ADT of 20,000 vpd is required, and so on.
More generally, the study showed that, put simply, the closer the work area is to the travel lanes, the more justified positive protection is. That is because there is a higher probability, with a closer proximity of motorist to worker, for conflict to occur. As the distance between the travel lanes and the work-zone area increases, the cost benefit of installing protective barriers decreases at lower volumes because of the reduced probability of an intruding vehicle reaching the work space and hitting a worker or construction vehicle/equipment.
Researchers also found that steel barriers and mobile barriers can be cost-effective alternatives for short-duration and mobile operations with workers on foot in high-volume, high-speed corridors. Given the short-term duration of deployment of these types of barriers, researchers identified projected cost savings per hour so that TxDOT can decide whether or not the project timeline justifies using these types of barriers for positive protection. As shown in Figures 3 and 4, even more expensive systems like the portable steel and mobile barrier systems can be justifiable over their service lives in terms of their crash-cost savings potential when used on higher-volume roadways.
Truck-mounted attenuators (TMAs) also were found to be very cost effective, offsetting their costs in less than a year of use in most cases. Researchers derived similar calculations for TMAs, and Figure 5 shows the results of that analysis.
“I feel that, in addition to making specific recommendations for when to install protective barriers, our real contribution in this project lies in generalizing the potential location of the worker or equipment throughout a given work-zone area,” said Ullman. “This makes our model more realistic and, therefore, more accurate when applied to real-world situations.”
Though developed specifically for Texas, the researchers’ findings can be applied anywhere a work zone exists. Like the research team, local traffic engineers can adapt RSAP to accommodate local societal costs associated with crashes and determine their own breakeven points for implementing positive protection in work zones. And that will help them stretch their maintenance dollars further while better protecting workers and motorists alike.
“Through this project, the researchers were able to determine when and in what conditions barrier protection is justified,” said Research Engineer Wade Odell of TxDOT’s Office of Research and Technology Implementation. “This is significant in that the contractor can provide motorists with proper protection in a work zone and still provide the protection cost effectively.” R&B
