Road Sign Performance Factors

Road Safety

3 Trends that Impact Road Sign Performance
- Video transcription:
  
  Despite the improvements in traffic safety over the past decade, several current trends reduce the ability of traffic signs to provide guidance and safety.
Headlight technology, illumination, and traffic sign performance
First, vehicle manufacturers have replaced conventional headlights with Visually Optically Aimable or VOA headlights. Designed to reduce the glare of oncoming vehicle headlights at night, VOA headlights also reduce the amount of light available to illuminate road signs.

To illustrate the amount of light that reaches signs mounted in various locations, let’s take a right-shoulder mounted sign—illuminated with conventional headlights—and assign it a benchmark illumination of 100%.

As you can see, even conventional headlights are only marginally effective at illuminating signs in “disadvantaged locations” such as the left shoulder and overhead.

With VOA headlights, sign illumination drops significantly. The same right shoulder-mounted sign is only 47% illuminated. And the overhead signs are just 6.6% and 8% illuminated.

In fact, one study found that VOA headlights suffered up to a 53% reduction in the light directed toward traffic signs compared to earlier, conventional headlights.
Additional insight

Since the late 1990s, vehicle manufacturers have transitioned to new headlight designs like Visually Optically Aimable (VOA) or low cut-off headlights. Designed as a solution to reduce the amount of glare drivers experience from oncoming traffic, these headlights have a sharp, horizontal cut-off and emit little light above the headlight level.
Low-cut off headlights have a profound effect on a vehicle’s ability to illuminate road signs. Recent headlight models do not provide as much illumination as did an average vehicle in 1997 for the most commonly viewed signs (on the right shoulder) at distances associated with typical sign reading. For a typical right-shoulder mounted sign viewed at distances between 300 feet and 900 feet, the reduction in illumination from 1997 to 2011 model headlights was anywhere from 24 percent up to 48 percent. ¹ With newer generation VOA headlights sold in 2004 through 2011 model vehicles, some disadvantaged locations (left shoulder and overhead signs) showed slight improvement in illumination. ¹ However, considering that a left shoulder mounted sign receives around 20 percent, and an overhead sign receives around 10 percent, of the illumination received by the right shoulder mounted sign, these signs are still very disadvantaged in terms of headlight illumination.

¹ Flannagan, M.J., and Schoettle, B., An Analysis of Low-Beam and High-Beam Headlighting Performance in the U.S.: 1997-2011, University of Michigan Transportation Research Institute, 2012.
Older drivers need more visible road signs and pavement markings
Another trend affecting traffic sign and pavement marking effectiveness is the growing number of older drivers on our roadways.

By 2020, nearly one in five drivers in the U.S. will be 65 or older and exhibiting the natural decline in sensory, cognitive and physical function that makes driving more of a challenge. These drivers need more visible signs and pavement markings to help retain their mobility.
Additional insight

In the U.S., nearly 8,000 baby boomers turn 65 every day—a trend that is expected to continue well into the future. By 2020, the U.S. population will include nearly 56 million citizens age 65 and older, and by 2030 that numbers swells to more than 72 million.
- In addition, older adults are keeping their driver licenses longer. In 2010, 80 percent of the 70-and-older population was licensed to drive—approximately 22.3 million seniors—and this number will continue to rise.¹
  
  As people age, there is a decline in many of the abilities considered necessary to safely operate a motor vehicle. Older people, as a group, have reduced visual acuity, narrower visual fields, poorer nighttime vision, greater sensitivity to glare, slower reaction times, more attention deficits, reduced muscle strength, reduced flexibility and range of motion, and other declines in visual, cognitive, and psychomotor function that can adversely affect driving.²
  
  Improving the visibility of road signs and pavement markings can be particularly important for older drivers who—between the ages of 60 and 80—need three to six times more light to see than a 20-year-old.³ Maintained traffic sign retroflectivity, larger signs with increased letter height, advance warning signs and street signs, improved road delineation with wider, well-maintained pavement markings and more traffic control in work zones are some of the recommended low cost safety improvements that can benefit older drivers.
  
  ¹Insurance Institute for Highway Safety, iihs.org/research/qanda/older_people.aspx
  ²National Cooperative Highway Research Program, NCHRP Report 500
  ³American Automobile Association, Inc., SeniorDriving.AAA.com
Vehicle size, observation angle and traffic sign visibility
When it comes to seeing traffic signs, the size of the vehicle you drive can also make a difference.

In a typical passenger vehicle, for example, the light from your headlights, striking a sign at 300-feet, will return to you at about a 0.5 degree angle. This is called the angle of observation.

In an SUV or pickup truck, this angle increases to nearly 0.6 degrees.

And in a semi-trailer truck where the driver is even more separated from the headlights, the observation angle expands to approximately 1.2 degrees.

The angle of observation is significant because signs must be visible for drivers of every vehicle type, at critical detection and reading distances along the approach.

If the cone of reflectivity is narrow, drivers of large vehicles will have difficulty seeing traffic signs at night.
Additional insight

The same sign may appear less bright to drivers of SUV s and large trucks than it does to drivers of passenger vehicles.

The number of large trucks registered in the U.S. has increased steadily over the last two decades, reaching almost 11 million in 2009. During that same time, truck traffic (as a percent of vehicle miles traveled) has doubled, nearing 300 billion miles in the U.S. in 2009. ¹ The FHWA reports that by 2040, long-haul freight truck traffic in the U.S. is expected to increase dramatically on the National Highway System and may reach 590 million miles per day. ² And, unlike automobile and business-truck traffic that peaks between the daytime hours of 7:00 a.m. and 7:00 p.m., long-haul truck traffic has a flat time-of-day distribution, with as many vehicles traveling during nighttime hours as during the day.³

When it comes to seeing traffic signs at night, drivers of large vehicles are at a particular disadvantage because of their observation angle is significantly greater.

How important is this measure? Large trucks can have twice the observation angle of passenger vehicles at the same distance. That means there’s much less reflected light available for these drivers to see road signs. Drivers of large vehicles can greatly benefit from signs made with high performance reflective sign sheeting that returns more light in a larger cone of reflectivity.
References
¹ Commercial Motor Vehicle Facts - November 2011, U.S. Department of Transportation
² FHWA-HOP-13-001 - Freight Facts and Figures, 2012
³ FHWA-RD-98-117 - Understanding Traffic Variations by Vehicle Classifications

3 Trends that Impact Road Sign Performance

Headlight technology, illumination, and traffic sign performance

Additional insight

Older drivers need more visible road signs and pavement markings

Additional insight

Vehicle size, observation angle and traffic sign visibility

Additional insight

References