U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Highway professionals use all available guidelines and principles to design roadways to maximize their functionality and safety. Yet traffic control devices are needed to delineate the alignment, assign right of way, and communicate regulations, warnings, and other information to drivers so they can navigate safely and efficiently, both day and night. More advanced communication tools are also becoming more available to augment traditional devices or to inform drivers of temporary or infrequent safety risks, such as work zones, congestion, and weather. As connected and automated vehicles become a larger portion of the fleet, the infrastructure is likely to change to communicate not only visually with the driver but also electronically with the vehicle.
The CMV drivers require special safety messages about the roadway: critical geometrics like bridge and tunnel clearances may interfere with oversize loads; curves, intersections, and grades require adjustments in speed, braking, or engine power. Operating characteristics of CMVs—longer braking distances, more gradual acceleration, and higher centers of gravity—make CMVs more susceptible to these variations in the roadway environment, both those that are normal and the unexpected (e.g., changes in weather that diminish visibility or risk rollovers, or sudden congestion in work zones). The following safety practices are particularly applicable for CMV safety.
Roadside signs convey a variety of messages to drivers—regulatory signs inform of the laws and regulations on the road and at intersections; warning signs inform of safety risks; guide signs aid wayfinding and direction; and service signs inform of nearby food, gas, lodging, or parking areas. The MUTCD provides the national standard for size, shape, color, and application of all traffic control devices, including signs and signals (discussed in this section) and pavement markings (discussed in the next section). Uniformity is a key factor in communicating so that drivers can quickly assess information through common sign formats, colors, and designs. The devices included in this section are placed on the roadside.
Diamond-shaped yellow signs with black symbols or legends warn drivers of a range of roadway conditions that may be unexpected; Figure 14 shows an example. These signs are installed when circumstances warrant (according to engineering studies and judgment) and stay in place as long as the roadway conditions are in effect. Static signs are used regularly because they are a cost-effective means of conveying needed information to drivers to allow them to safely negotiate an unfamiliar roadway.(27) Signs that are particularly beneficial for safety of CMVs and preservation of the infrastructure include:
STOP ahead, YIELD ahead, or SIGNAL ahead warning signs that provide the additional time needed for larger vehicles to begin slowing before the intersection control device itself comes into view.
Source: FHWA, MUTCD, Figure 2C-1(43)
Figure 14 Graphic. Truck rollover warning sign.
Static warning signs run a risk of losing effectiveness for frequent travelers on a roadway because drivers may grow accustomed to signs as background scenery. This can be a risk for CMV drivers following regular urban or intercity routes. Sometimes, these static signs can be increased in size to increase their conspicuity. In corridors with significant truck traffic, placing a duplicate sign on the left side of the road can also provide drivers of passenger vehicles a better opportunity to see signs that may otherwise be blocked by large vehicles in the right lanes. Static signs, both in standard and larger format sizes, have proven to be a cost-effective method of communicating roadway conditions to CMV and other drivers.
Cost-Effectiveness Assessment: Static warning signs are assessed as proven (static) or experimental/promising (oversized static), and low cost.
The FHWA oversees a process for updating the MUTCD on a periodic basis to better reflect the most current knowledge on good practice, based primarily on research and in-service evaluations. Most updates to the MUTCD do not have a specific time frame but rather are to be completed through a systematic approach or when the existing device is replaced because it is no longer serviceable. Because it may take a long time for updates to be seen across the entire roadway network, when there is a sufficient safety implication to justify it, FHWA specifies a timetable for compliance with the new standards and provisions. Two such changes have implications for CMV safety.
A rulemaking in 2007 established minimum maintained standards for retroreflectivity of traffic signs. Retroreflectivity of traffic signs is crucial to drivers' ability to see the signs at night, especially in areas without continuous roadway lighting. Because the angle from the vehicle headlamps to the sign and back to the driver's eye is greater for large vehicles, signs are less visible at night to drivers of these vehicles, making adequate retroreflectivity even more critical. As more signs with deteriorated retroreflectivity are replaced in response to the standards, CMV drivers should be better able to see traffic signs at night (Figure 15).
Source: FHWA, Low-Cost Treatments for Horizontal Curve Safety.(44)
Figure 15 Photo. Retroreflective chevrons.
A more recent change to the MUTCD focuses on use of warning signs for horizontal curves. The 2009 MUTCD established new standards and guidance to improve the consistent use of advance warning signs, advisory speed plaques, and chevrons at horizontal curves. The changes, encompassing sections 2C.06 through 2C.08, also encourage methods for setting advisory speeds that are more comparable to design practices than many agencies have used in the past. Highway agencies determine advisory speeds for horizontal curves to maintain necessary side friction for safety of negotiating the curve and for driver comfort. Depending on the difference between posted and advisory speeds, MUTCD table 2C-5 establishes the signs that are required, recommended, or optional. Updating the sign infrastructure to these new standards will provide a consistent message to drivers as they drive on highways operated by different agencies and may help address the more than one-quarter of all fatal crashes that are associated with horizontal curves.(44) The CMV risks at horizontal curves are greater than the passenger vehicle risk due to their mass and high center of gravity.
Cost-Effectiveness Assessment: Updating signs to MUTCD standards is assessed as proven and moderate cost.
To improve the chances of drivers noticing important signs, highway agencies sometimes use dynamic signs with flashing lights or beacons to capture driver attention. A variation on this uses detection systems to monitor traffic and flash or provide other specific information that relates to the roadway or traffic condition of concern when triggered. One example is height detection equipment in advance of bridges with clearance restrictions. Another is dynamic speed display signs, which combine speed detection systems displaying the vehicle's actual speed along with a static sign with the regulatory speed limit or advisory speed (Figure 16).
Source: FHWA, Low-Cost Treatments for Horizontal Curve Safety. Chapter 4.(44)
Figure 16 Photo. Dynamic speed signs.
Some dynamic signs only flash or display a message if the speed detection unit indicates the vehicle is approaching over a set speed. For example, sequential flashing chevrons provide more active warning to drivers who need to slow down to safely negotiate the curve. Unfortunately, reductions in speeds found with these devices often do not last after drivers become accustomed to them, and a reduction in crashes has been difficult to determine The ability of various size vehicles to safely negotiate a curve with the same geometrics and pavement friction complicate the research efforts. However, since larger CMVs are more sensitive to these factors, the devices are likely more beneficial for these vehicles.
Another type of dynamic sign displays a driver message based on a broader range of vehicle behavior (speed, weight, or height) in advance of high risk areas for CMVs—tight curves, restricted clearances, load-zoned bridges, or signalized intersections (Figure 17). These dynamic systems can reduce rollover or run-off-the-road crashes by reducing speeds at curves and can reduce congestion-related crashes when oversize vehicles cannot move beyond restricted bridge or tunnel clearances. (See references 44, 45, 46, 47, 48, 49, and 50) These signs may need further improvements to guard against frequent traveler inattention.
Figure 17. Photo. Dynamic warning sign showing activated truck rollover warning system.
Cost-Effectiveness Assessment: Dynamic warning devices are assessed as proven and low to moderate cost.
Because CMVs are more difficult than smaller vehicles to stop due to their weight, they can pose risks at signalized intersections. A traffic signal control system has been designed and tested that continually monitors vehicles approaching an intersection to determine the best time to end the green phase for the major road, allowing all vehicles, but especially CMVs, a better opportunity to proceed safely through the intersection rather than run a light or stop suddenly and risk a crash.(51) The system measures vehicle speed, vehicle length, and the lane that each vehicle occupies and predicts their arrival based on detection at 1,000 feet from the intersection. Research findings based on full-scale testing in several States indicate significant improvements in metrics related to safety such as red-light running, vehicles caught in the dilemma zone, and maximum green light timing cycles automatically ending a green phase regardless of the vehicles in the intersection. This countermeasure may be effective for signalized intersections along arterials that are part of State-designated priority freight corridors.
Cost-Effectiveness Assessment: Detection-control systems for traffic signals are assessed as proven and moderate to high cost.
Another set of safety-related communication treatments involves markings on the pavement because drivers regularly watch the pavement ahead for lane directions and possible obstructions. The primary function of markings such as lane lines, center lines, and edge lines is to assign space on the paved roadway to vehicles traveling in the same or opposing directions, but these markings also provide the driver information on alignment changes and passing opportunities and regulations. Other markings provide additional information such as the location of turn lanes or marked pedestrian crossings.
The retroreflectivity of pavement markings is critical to nighttime drivers because the pavement can be difficult to see more than a short distance ahead with headlamps alone. Unlike the situation with signs, drivers of CMVs are actually better able to see retroreflective markings than drivers of smaller vehicles. This is because the markings are on the pavement in front of the driver whereas signs are off to the side and several feet above the pavement. Larger vehicles are equipped with higher-intensity headlamps that are mounted higher than those on automobiles, which is more important in this situation. However, increasing the retroreflectivity of pavement markings will aid in the visibility of the markings during nighttime hours for all drivers.(52,53) The following treatments also offer proactive communications to drivers, including CMV drivers, to allow more fluid, more predictable auto-truck interactions along the road.
Horizontal signing refers to messages typically communicated by signs on the roadside or overhead being placed on the pavement in addition to the sign. (Figure 18). It can include route numbers, stop and yield markings, traffic or parking prohibitions, bus lane markings, or school zone markings. Supplementing signage with markings is helpful to all drivers. One case that is particularly beneficial for larger vehicles is route signing at interchanges because it may reduce risky behavior by passenger vehicle drivers. These passenger vehicle drivers may make last-minute lane changes, often immediately in front of a CMV that has intentionally left a larger gap to enable the longer stopping distance needed. Research has shown the kinds of horizontal signage that most effectively communicate to drivers, but more research may be needed to demonstrate that such signs have a positive impact on CMV crashes. (54,55,56)
Source: Virginia Department of Transportation. Use of Horizontal Pavement Markings.(57)
Figure 18. Photo. Route shield pavement marking in Virginia.
Cost-Effectiveness Assessment: Horizontal signing is assessed as tried and low cost.
Wider edge lines are any marking along the outside pavement edge over the 4-inch minimum MUTCD standard (Figure 19). Studies have shown that 6-inch edge lines more effectively communicate usable lane widths along two-lane roads.(58,59,60) Many of these studies examine general driver behavior but do not examine the effects of wider edge lines on CMV crashes. However, fatal crashes involving large trucks occur more frequently on rural roadways than urban, so this treatment may have some applicability for CMV safety. Wider edge lines have a proven safety benefit and are a cost-effective crash countermeasure. They have also been found to improve operational metrics such as encroachment and driver eye glance when compared to pavement marking brightness.
Cost-Effectiveness Assessment: Wider edge lines are assessed as proven and low cost.
Source: Carlson, P. and Wagner, J. An Evaluation of the Effectiveness of Wider Edge Line Pavement Markings.(58)
Figure 19 Photo. Wider edge line marking.
On concrete surfaces, pavement markings may blend with the concrete color during the daylight. To improve lane marking visibility, black contrast marking material is applied adjacent to the retroreflective white or yellow line, or beneath the standard marking with a larger footprint, as shown in Figure 20. This type of pavement marking increases the conspicuity of longitudinal pavement markings, with possible benefits for fatigued CMV drivers.(61,62)
Source: Brewer, M., Murillo, D., and Pate, A. Handbook for Designing Roadways for the Aging Population. (63)
Figure 20. Photo. Contrast marking.
Cost-Effectiveness Assessment: Contrast markings are assessed as tried and low cost.
This final class of safety communication treatments addresses CMV safety in higher-risk environments that may be non-recurring or temporary. Risks are associated with changes in lane alignment or configuration, aggregate traffic patterns that change, or weather-related conditions that pose risks to CMV operations. Two are focused on operational safety issues within work zones because these are related to construction that provides for the upkeep of the infrastructure. As mentioned in chapter one, nearly a third of all work zone traffic fatal crashes involve at least one truck. The FHWA has been leading a joint effort with other USDOT agencies over the last several years to improve large truck safety in work zones. The final countermeasure included in this section relates to weather conditions that can pose CMV safety challenges on certain segments of the infrastructure.
This system uses vehicle speed monitoring to detect queue buildup at critical transition points in work zones and displays real-time warnings to upstream motorists using portable changeable message signs. Drum-mounted radar systems can be deployed at the beginning of work-zone-related constrictions or alignment changes, and can be moved as work zones change during different project phases (Figure 21). This system offers promise for CMV drivers in work zones because 30 percent of work zone crashes are related to large trucks. Displaying queue detection to drivers can help CMVs begin to slow down in advance, reducing the risk of not being able to stop before they reach the queued traffic. This is one of several strategies that can be found on the "Large Trucks in Work Zones" Web page at the National Work Zone Safety Information Clearinghouse (https://www.workzonesafety.org/work_zone_topics/heavy-vehicles/).
Brydia B. Getting information to drivers to improve awareness, safety.(64)
Figure 21 Photo. Dynamic queue detection warning sign in a work zone.
Cost-Effectiveness Assessment: Queue detection warning in work zones is assessed as proven and moderate to high cost.
With the exception of very small owner-operators, most commercial motor carriers communicate to truck drivers through dispatchers, who assign loads and monitor truck location and condition. Larger motor carriers also connect to CMVs through satellite or wireless networks that provide real-time tracking of vehicles and shipments, engine diagnostics and fuel use, hours of service logging, and driver communication. The FMCSA is seeking to take advantage of these CMV driver communication networks to connect to public information about work zone activities. The FMCSA Innovative Technology Deployment grant-funded projects are implementing an in-cab alert system that will notify the CMV driver of an active work zone, traffic congestion, or an incident ahead. The alert system must be able to be broadcast through the CMVs' transponders, electronic onboard systems, cell phones, or motor carrier routing and dispatching systems. These notification systems will alert CMV drivers of work zone delays with sufficient advance notice to allow dispatchers to suggest alternate routes.
Cost-Effectiveness Assessment: Work zone and incident electronic notification systems are assessed as experimental/promising and moderate to high cost.
Limited visibility due to fog, dust, or smoke can lead to infrequent but high-consequence crashes, sometimes involving multiple vehicles, including CMVs. High winds affect large vehicle dynamics, especially tractor-trailer combinations. Road Weather Information Systems are used in locations with frequent weather risks to inform drivers about adverse road conditions during weather events. The message conveyed may be general (Figure 22) or may include specific suggestions such as adjusting behavior (e.g., reduce speed) or vehicle performance (e.g., use tire chains), or it may suggest drivers stop driving or seek alternate routes due to road closures. In some cases, the road closure may be specific to high profile vehicles (Figure 23).(65) Some States combine these weather detection systems with variable speed limits to further reduce risk. These systems monitor weather conditions, either from a central location or with local weather detection measurement devices, and communicate adverse weather conditions through dynamic message signs. These warning systems can provide valuable information to CMV drivers to reduce exposure to weather risks.
Cost-Effectiveness Assessment: Visibility and wind detection systems are assessed as proven and moderate to high cost.
Source: Elkins, H. B. Millennium Highway: The New I-26 Virtual Tour.(66)
Figure 22 Photo. Weather advisory system using a static sign with beacons.
Source: Orr, B. "Wyoming wind not abnormal but the timing is." Photo Credit: Blaine McCartney, Wyoming Tribune Eagle(67)
Figure 23 Photo. Weather advisory system with a changeable message sign.