U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
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Approximately 90 percent of the information that drivers use is visual.(1) For nighttime driving, two of the most important factors that affect vision are visual acuity and contrast sensitivity. Visual acuity is the ability for drivers to discern details at a distance. The ability to detect differences in luminance and discern an object from its background is contrast sensitivity.(2)
A visual acuity of 20/20 is considered normal vision. In ideal conditions, a person with normal vision can read letters that subtend an angle of 5 minutes of arc. A person with 20/40 vision needs letters of 10 minutes of arc. A driver with poor vision must be closer to signs or roadway hazards to see them.(2) Contrast sensitivity is noted as being more impactful on vehicular crashes than is visual acuity. Regardless of a driver’s visual acuity, an object that cannot be distinguished from its background will not be seen. This could involve any number of roadway hazards, from debris to a crossing pedestrian. Drivers typically misjudge the distance at which pedestrians are seen,(3) which often leads to crashes. The use of lighting can increase contrast in most cases and make objects in the roadway more discernible.(2)
For pedestrians and bicycles, it is important that conspicuity be increased to promote visibility. Lighting is one way to increase conspicuity. Although street lighting can help drivers see these vulnerable road users, a lack of uniformity can create dark spots, improper lighting can create glare, and changes in asphalt color can create areas of poor contrast.(2)
Color vision is a result of three types of color-sensitive cones in the eye. These three cones respond in general to red, green, and blue components of the visual spectrum. Abnormal color vision (or color blindness) refers to an aspect of vision where shades of color are confused by the observer. This is typically a result of low or missing levels of pigment in one of the cones in the retina. Color deficiencies can be red confusion, green confusion, or blue/yellow confusion (Protanopia, Deuteranopia, and Tritanopia, respectively). Color blindness occurs in approximately 8–9 percent of the population and mostly in males. Colorblind drivers typically use position of traffic signals and other clues to overcome their disability. There are new considerations in universal design that can overcome the limitations of color vision. These approaches are similar to a blue cross embedded in the red signal to aid in the signal identification.
The FHWA Lighting Handbook discusses the decline of visual function that often comes with age. Typical aging issues include a reduction in pupil size, which reduces the amount of light entering the eye; the lens losing elasticity, making focus difficult; and a general yellowing of the lens, which reduces both luminance and color contrast. Diseases such as glaucoma affect peripheral vision, and the yellowing of the ocular lens diminishes contrast sensitivity. The handbook also states that people over 60 have a reduced ability to detect movement of objects. Older drivers are also more susceptible to glare because the amount of contrast needed to overcome threshold contrast increases significantly. The concerns of the aging population’s vision extend beyond drivers to pedestrians as well. Pedestrians, like drivers, need to accurately read sign messages and detect motion. Even though these concerns are known, the current roadway lighting system design practices are not designed with modifiers for age.(4)
In the FHWA Safety Program, a roadway departure (RwD) crash is any crash in which a vehicle crosses an edge line, crosses a center line, or otherwise leaves the traveled way. Roadway departures account for over half of fatal crashes in the United States.(5) Harmful events associated with fatal roadway departures are fairly broad; however, the most harmful event in over 70 percent of crashes are rollovers, collisions with other vehicles and collisions with trees. Various factors come into play with roadway departures, including road type, speed limit, roadway geometry, weather, distracted driving, and even pavement. This section contains common practices for lighting various roadway types and concludes with potential areas of research for visibility and lighting that can help prevent roadway departures.
The strategic narrative for the FHWA roadway departure safety roadmap focuses on the goal of reducing roadway departure fatalities by 50 percent by the year 2030. The principal strategy is a focus on data, and there are some summary statistics of characteristics of roadway departure fatalities. For example, 76 percent of roadway departure rollover fatalities and 70 percent of roadway departure opposite-direction fatalities occur in rural areas. Also, over 70 percent of rural fatal crashes occur with a posted speed limit of 50 mi/h or greater.(6)
Data for the proportion of roadway departure fatalities that occur at night are notably absent from the strategic narrative. It is well known that nighttime crash rates are disproportionately high, especially for roadway departures. Therefore, any treatment that specifically targets nighttime crashes has a great potential to significantly improve safety. There is little within the strategic narrative that emphasizes the importance of these topics.
The strategic narrative mentions improved delineation as a potential countermeasure and the FHWA Lighting Handbook and training as a resource for addressing roadway departure safety.(6) There is limited information about how visibility will be integrated into the future of the roadway departure program area. Online, however, there is more information about retroreflectivity, lighting, and visibility within the current roadway departure safety program. That information relates to Manual on Uniform Traffic Control Devices (MUTCD) regulations, research, funding for visibility improvements, and other resources for implementing the objectives of the program. In general, however, there is not a direct link between these visibility topics and safety. Without evidence of a direct link, it may be difficult to persuade agencies to change their practices and policies.
Along the same lines, the national guides have found deficiencies in design practice. The lighting needs for different roadways are highlighted in the American Association of State and Highway Transportation Officials’ (AASHTO’s) lighting guidance in A Policy on Geometric Design of Highways and Streets (Green Book).(7) Because fixed-source lighting tends to reduce crashes, and most crashes occur at critical points such as interchanges, curves, and junctions, AASHTO recommends lighting many urban and suburban areas. Rural lighting is rarely justified except for critical areas like intersections, railroad crossings, bridges, or sharp curves.(7)
There are several issues that result in a difference in design between rural and urban zones. Rural areas may have higher speed limits due to fewer conflicts such as intersections, curbs, and pedestrians. The levels in background luminance may be different since by definition rural areas are less populated, so there will be less surrounding illumination.(8)
Within the area of roadway departure safety, there are several research projects that have recently been completed or are ongoing that relate to visibility and the use of traffic control devices and lighting. Examples of these are described below.
Pavement markings provide roadway alignment information that helps drivers maintain their position in the lane and identify upcoming features such as curves and ramps. Recent research conducted for the Texas Department of Transportation and FHWA has identified the benefits of using wider (greater than 4 inch) edge lines,(9,10,11) which leads to improvements in lane position, reductions in encroachments across the edge line, and reductions in observed crashes. A key finding from the work on wider edge lines is that they are shown to significantly reduce crashes on two-lane highways but not on multilane divided highways (which is where they are generally used). A pooled fund study recently showed that upgrading pavement markings so they are retroreflective when wet can result in significant crash reductions for multiple types of crashes, especially on multilane highways.(12)
In addition, there have been recent studies on how pavement marking retroreflectivity may be associated with nighttime crashes (see 13, 14, 15, 16, and 17). While there is not yet an established link that has been widely accepted, research has been making progress on identifying thresholds of pavement marking retroreflectivity where improvement in nighttime safety may be achieved.
Complementing the research on wider and brighter pavement markings and on markings designed for performance in wet weather, the Texas A&M Transportation Institute (TTI) is currently leading a project funded through the National Cooperative Highway Research Program (NCHRP) on retroreflective pavement markers (RPMs). RPMs provide the most notable benefit during wet weather events at night because their construction and shape prevent water from accumulating on the marker. The objectives of the NCHRP project include addressing where RPMs should be used and developing specifications for spacing and maintenance (i.e., retroreflectivity and retention).
Regarding traffic signs that can prevent roadway departure crashes, a recently completed NCHRP project developed guidelines for the use of traffic control devices at changes in alignment.(18) Researchers evaluated the safety impacts of traditional devices used at curves (such as advance warning signs and chevrons) and studied how drivers respond to these devices. From the findings, they developed guidelines that promote the consistent use of devices that provide drivers the right information for safe curve negotiation.
Regarding lighting, the Virginia Tech Transportation Institute (VTTI) has recently completed nighttime visibility projects involving adaptive lighting, light-emitting diode (LED) sign board visibility in fog, and fog and rain visibility with active lane delineators. The adaptive lighting project produced guidelines for adaptive roadway lighting design criteria.
The LED sign board visibility in particular was developed to determine the best color combinations for signs in foggy conditions. These will be used in areas where adaptive speed limits based on weather conditions are implemented. The results indicate that white and black remain the best combination.
The active delineators have shown promise in providing information to control both speed and roadway position of a driver in fog. A variety of flash patterns were tested to investigate the impact on driver behavior. The results show that active delineators can provide lane guidance as well as speed control in low visibility conditions.
Currently in progress at VTTI are research projects concerning the impact of roadside police vehicle and active work zone equipment lighting on visibility. These projects will produce recommendations for equipping vehicles with lighting that promotes traffic and worker safety. The Lighting Research Center and Penn State identified links between visibility and public safety from roadway lighting in a recent research effort. The research was prompted by a recent trend of municipalities removing their lighting in an effort to save money and energy. While lighting is expected to improve safety, little data are available to serve as a link between the two ideas. This research effort attempted to solidify that link and provide information on the consequences of removing or adding lighting to an area.
An intersection crash is any crash where an intersection or driveway access is identified as the location or one of the influencing factors. Intersections vary widely depending on location, functional class, traffic volume, and use of channelization. While fatal crashes at intersections have declined in recent years, determining how and why crashes continue to occur is still a strong research focus. Most research shows that the addition of roadway lighting and other visible elements can reduce crash frequency at an intersection.
One of the program strategies for intersection safety is a prioritization of efforts based on data. While fatalities have steadily dropped in recent years, it is clear that continued improvements will be best realized by focusing on the conditions in which crashes occur most frequently. The strategic plan focuses on both signalized and unsignalized intersections, with additional emphasis areas categorized as vulnerable users: pedestrians, bicycles, motorcycles, and older drivers.
Some analysis has been performed for each of these emphasis areas, showing trends and summary statistics. There are also lists with potential treatments, projects, resources, and opportunities for coordination with other areas. For example, the areas for pedestrian and bicycle safety identify road diets and pedestrian-oriented roundabouts as potential methods to facilitate reductions in pedestrian and bicycle fatalities.
Of the topics and practices discussed in the strategic plan for intersection safety, there is only one mention of conditions related to visibility at night. Retroreflective signal backplates are listed as an effective countermeasure at signalized intersections. There are other projects that may have a component of visibility such as the recently completed “Accelerating Roundabout Implementation in the US” however there are few that deal with visibility or lighting as the explicit research focus.
In terms of the lighting, the primary goal is to provide sufficient illuminance so drivers can see their entry and exit points ahead of time. Other goals of intersection lighting include alerting drivers of an intersection ahead of time as well as providing comfort and safety for pedestrians. Many policies provide information on when intersection lighting is warranted, but specifics on implementation can be rare. Often left out of consideration are rural intersections, which are often dimly lit or unlit and designed for higher speeds and typically have other confounding issues such as dark sky considerations and availability of utilities. Currently, guidelines for roundabout lighting is based on policies for signalized or all-way-stop intersections enhanced with additional computer based rendering simulations although roundabouts and intersections are significantly different in their operation and visual areas of concern. Note that some intersections are not controlled in any of the directions, and lighting may impact the safety at these intersections with noticeability of the intersection and potential conflicts.
There may not be a need to include visibility and nighttime conditions as a separate emphasis area in the intersection safety strategic plan, but the typical disproportionate crash rates at night on all roadways justify at least identifying the resources and potential countermeasures that can address safety issues related to nighttime conditions for each of the emphasis areas.
Visibility within the focus area of intersection safety can be impacted by the use of lighting and traffic control devices at intersections. Several recent and ongoing projects investigating these issues are discussed here.
One key concern for safety at intersections is communicating to drivers the presence of intersection control so they properly respond. While the visibility of traditional intersection control devices such as stop signs and traffic signals is adequate in most cases, in some cases the intersection control devices can be made even more visible (or conspicuous) by measures such as added flashing lights, with the use of either beacons or LEDs embedded in stop signs.(19,20) Improving the sign visibility at the intersection may be another way to obtain similar results. FHWA research has shown that while upgrading stop sign retroreflectivity may lead to only a modest reduction in crashes, the low cost of installing new stop signs produces cost-effective benefits, particularly at lower-volume intersections.(21)
Research completed for an NCHRP project on overhead signs showed that legibility is directly linked to sign luminance, whether provided from internal illumination or with retroreflective sheeting.(22) Increase in retroreflectivity of signs or use of internal illumination can serve to not only enhance legibility but also improve overall conspicuity for alerting drivers to the intersection. TTI is currently evaluating the safety impacts of systemically upgrading signs with a case study in Albuquerque, New Mexico.
A recent study of lighting levels at isolated rural intersections was completed by the University of Minnesota. The study found that lighted intersections had reduced nighttime crashes compared to unlit intersections of comparable traffic volume and configuration. Increasing the horizontal illuminance is effective at reducing nighttime crash rate regardless of source, whether high-pressure sodium (HPS) or LED.(23) Other recent research found lighting at intersections to reduce crashes by 3.6–6.5 percent.(24)
In late 2015, VTTI completed a study investigating the positioning of intersection lighting and how that lighting helps promote the visibility of pedestrians. The study found that lighting the box of the intersection provided the best results in terms of visibility with a plateau of illuminance levels between 8 and 12 lx. This configuration requires fewer luminaires, which may lead to an energy saving solution.(25)
Differences in size, speed, and use of safety gear make pedestrians and cyclists the most vulnerable of road users. In fact, pedestrians accounted for approximately 18 percent of all traffic fatalities in 2014.(26) Nearly 70 percent of all pedestrian-related crashes occur at night. There are many policies in place for providing lighting to intersection crosswalks and midblock crosswalks, but typically with little regard for the differences in vehicle headlamps and surrounding illumination.
The strategic plan for the pedestrian and bicycle safety area, updated in 2010 and available online, discusses the proportion of nighttime fatal crashes that involve pedestrians. Sixty-nine percent of those fatal crashes occur at night. Reduced visibility compared to daytime conditions is clearly a contributing factor in the high number of pedestrian fatalities. The issue of nighttime visibility is clear in the strategic plan for pedestrian and bicycle safety, which lists several FHWA products and topic areas that address pedestrian safety at night. Some of these resources include the Pedestrian Forum Newsletter, the website pedbikeinfo.org, and information for developers to create safe and accessible communities. Other areas of the strategic plan summarize research related to nighttime visibility, such as the effects of lighting on pedestrian crashes.
FHWA sponsored some work assessing crosswalk lighting. In general, based on the level of pedestrian conflict, more lighting is required in areas where pedestrian conflict is greater. Crosswalks are often co-located at intersections, so intersection lighting is often designed to encompass sufficient lighting for crosswalks and pedestrians. Midblock crosswalks can be lit but are at location on a roadways in areas where vehicles do not naturally stop and therefore have different visibility requirements. Pedestrians are more dependent on their own judgement to keep them safe. Since many pedestrians do not cross strictly at marked crosswalks, some consideration must be made to allow for these potential conflicts.
Pedestrian safety from a perspective of visibility focuses on the ability of drivers to see pedestrians and to properly respond to their crossing at an intersection or potentially anywhere in the roadway. In a recent study, a collaborative effort between VTTI and North Carolina State University studied the gaze behavior of drivers entering and exiting roundabouts. Because roundabouts are features that allow vehicles to pass through the intersection without stopping, the ability to see crossing pedestrians at the right time is critical to their safety. Information gathered from this study can inform where pedestrian crossings and bicycle pathways may be best implemented.(27)
New methods that help drivers identify crossing pedestrians include pedestrian hybrid beacons (PHBs) and rectangular rapid-flashing beacons (RRFBs). RRFBs and PHBs are user-activated devices that help notify drivers that pedestrians are crossing or are about to begin crossing. Research has shown that drivers are more likely to yield when these treatments are used.(28,29) Recent research evaluated drivers yielding to pedestrians when these treatments are used to identify the conditions for which their use may be preferable.(30)
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