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Public Roads - March/April 2016

March/April 2016
Issue No:
Vol. 79 No. 5
Publication Number:
Table of Contents

Communication Product Updates

Communication Product Updates

by Lisa A. Shuler of FHWA's Office of Corporate Research, Technology, and Innovation Management

Below are brief descriptions of communications products recently developed by the Federal Highway Administration’s (FHWA) Office of Research, Development, and Technology. All of the reports are or will soon be available from the National Technical Information Service (NTIS). In some cases, limited copies of the communications products are available from FHWA’s Research and Technology (R&T) Product Distribution Center (PDC).

When ordering from NTIS, include the NTIS publication number (PB number) and the publication title. You also may visit the NTIS Web site at to order publications online. Call NTIS for current prices. For customers outside the United States, Canada, and Mexico, the cost is usually double the listed price. Address requests to:

National Technical Information Service
5301 Shawnee Road
Alexandria, VA 22312
Telephone: 703–605–6000
Toll-free number: 1–888–584–8332
Web site:

Requests for items available from the R&T Product Distribution Center should be addressed to:

R&T Product Distribution Center
Szanca Solutions/FHWA PDC
700 North 3rd Avenue
Altoona, PA 16601
Telephone: 814–239–1160
Fax: 814–239–2156

For more information on R&T communications products available from FHWA, visit FHWA’s Web site at, the FHWA Research Library at (or email, or the National Transportation Library at (or email


Strength Characterization of Open-Graded Aggregates for Structural Backfills (Report)

Publication Number: FHWA-HRT-15-034


Open-graded aggregates are compacted aggregates with relatively large void spaces not filled with intermediate-sized particles. These aggregates are becoming more common in road and bridge construction because they are easy to place, require simple testing for quality assurance, and have the advantages of very low fine particle content, lighter unit weight, free-draining characteristics, and low potential for frost heave.

State and local transportation agencies frequently use crushed, manufactured open-graded aggregates as structural backfill material for retaining walls, bridge foundations, and other ground improvement applications. However, the strength characteristics of this material are not fully understood or applied. Designers need to know the friction angle to account for lateral earth pressures and bearing resistance. But because of the large size of the standard American Association of State Highway and Transportation Officials (AASHTO) open-graded aggregates, this parameter cannot be measured with standard testing equipment. Instead, current practice is to select aggregates with low default friction angles, which can lead to overly conservative, less cost-effective designs.

This report discusses a study to establish a knowledge base featuring the most commonly used AASHTO open-graded aggregates. The study included a systematic approach to fully characterize the strength parameters using a large-scale direct shear and triaxial device in FHWA’s geotechnical laboratory at the Turner-Fairbank Highway Research Center (TFHRC). The researchers also investigated relationships between other important soil parameters and the friction angle, as well as the effect of various automated testing devices and methods to interpret the data.

The report presents strength characteristics of 16 open-graded aggregates representative of commonly selected structural backfills. The test results indicate higher strengths for these materials than current default values typically assumed in design. Aside from strength, researchers measured gradation, density, repose angle, angularity, and texture. They also examined correlations between various soil properties and strength parameters and found that the mean grain size, sphericity, angularity, and void ratio play a role in the measured friction angles.

The document is available to download at


Evaluation of the Impact of Spectral Power Distribution on Driver Performance (Report)

Publication Number: FHWA-HRT-15-047


Traditional roadway lighting uses high-pressure sodium light sources, which provide high photometric efficacy. However, this type of light is amber and does not render the color of objects accurately. With the advent of light-emitting diode technology in roadway lighting, a new aspect of a light source is now being considered--its spectral power distribution. Broad-spectrum sources, with significant spectral output across the entire visible spectrum, potentially provide additional benefits to the driver; these light sources can provide better color information and can activate all of the photoreceptors in the eye more efficiently. This report discusses a project that investigates these effects and considers the potential benefits of a broad-spectrum light source.

The potential benefits relate to mesopic vision--the transition in the human eye from cone sensitivity to rod sensitivity in lower light situations. In a series of human factors experiments, researchers evaluated the effect of the spectral power distribution of overhead lighting and headlamps with respect to driver detection and recognition of large and small objects as well as pedestrians. The report includes an evaluation of the effects of a momentary peripheral illumination mechanism on visual performance and eye-glance behavior of drivers. Researchers also evaluated the potential for applying mesopic multiplying factors to roadway lighting.

Results of these experiments show that in a natural driving environment at the speeds tested, there is limited applicability of the mesopic model to lighting design. Researchers found that the spectral component of the light source affects driver visual performance, but only in certain conditions. The speed of the vehicle also affects the driver’s visual performance. For high-speed roadways, researchers recommend that spectral effects not be included in the design of the lighting systems. For lower speed roadways where the lighting system is predominantly for pedestrians, spectral effects may still apply.

The report is available to download at


Properties of Anchor Rods Removed From San Francisco-Oakland Bay Bridge (Report)

Publication Number: FHWA-HRT-15-057


In March 2013, the construction contractor for the new self-anchored suspension bridge between San Francisco and Oakland, CA, tensioned the threaded rods between the bearings and shear keys and the concrete pier cap. Soon after completion, one-third of the rods for the two shear keys on the eastern pier fractured. This report discusses the investigation, requested by the FHWA California Division, to determine the root cause of the fractures. The document includes data on the mechanical, chemical, and microstructural properties of two samples removed from the pier.

The two samples were shipped to TFHRC. One was from a threaded portion of a rod and was approximately 3 inches (7.6 centimeters) long. The second sample was from an unthreaded portion and was approximately 13.5 inches (34.3 centimeters) long. Researchers at TFHRC were asked to test impact toughness, hardness, strength, and elongation, in addition to conducting microscopic inspection of thread roots for cracks.

Testing showed a variation in material properties between the surface and core of the rods. Researchers concluded that improper heat treatment of the rods caused this variation. In addition, the tensile and hardness properties could have been judged to be in nonconformance depending on interpretation of the relevant ASTM standards. Researchers recommend that the ASTM A354 and/or F606 standards be revised to provide more guidance on sampling for tensile and hardness properties for large-diameter products as well as guidance on impact toughness.

The report is available to download at


Assessing Pavement Surface Splash and Spray Impact on Road Users (TechBrief)

Publication Number: FHWA-HRT-15-062


The effects of vehicle splash and spray are well known to motorists who have driven in wet weather conditions. Research indicates that splash and spray contribute to a small but measureable portion of vehicle crashes and are sources of considerable nuisance to motorists. Splash and spray from highway pavements can also carry a number of pollutants and contaminants. When deposited on roadsides, these contaminants can be detrimental to plant life and accelerate the corrosion of roadway appurtenances. This technical brief describes the development of an assessment tool to characterize the propensity of highway sections to generate splash and spray during rainfall and the impact of splash and spray on road users.

The development of the assessment tool involved a combination of laboratory and field experiments, theoretical developments, and computer simulations. Researchers assessed prior work in the area of splash and spray mechanisms and found that the main factors affecting splash and spray are water film thickness, vehicle speed, tire geometry, tire tread depth, vehicle aerodynamics, and devices to suppress vehicle spray. They also found that none of the techniques that have been used to measure splash and spray are widely accepted or readily available for routine evaluations of pavement surfaces. A review of the available literature suggests that there are four prevailing splash and spray mechanisms: bow wave, side wave, tread pickup, and capillary adhesion.

Researchers developed three submodules for the splash and spray assessment tool: (1) a water film thickness model, (2) an exposure model for estimating the amount of water that will be projected by the tire, and (3) a splash and spray model that predicts the likelihood of the occurrence of splash and spray based on the other two models.

The study’s submodels comprise a model that can be used to predict splash and spray based on the characteristics of a pavement surface and climatic conditions. The model computes water film thickness based on rainfall intensity and pavement surface properties, the maximum amount of water available for splash and spray, and the contribution of each splash and spray mechanism until the total amount of available water is exhausted. The model can also determine the spray density corresponding to each mechanism and the total spray density, and convert the spray density level to a subjective nuisance index. These computations can provide useful information for supporting highway design and maintenance processes.

The technical brief is available to download at


Breakthroughs in Vision and Visibility for Highway Safety (Workshop Summary Report)

Publication Number: FHWA-HRT-15-067


On August 13 and 14, 2014, FHWA’s Office of Safety Research and Development and Office of Safety, with support from the Exploratory Advanced Research Program, convened a 2-day workshop at TFHRC. The workshop, “Breakthroughs in Vision and Visibility for Highway Safety,” aimed to identify gaps in highway visibility research, explore innovative tools and techniques to fill those gaps, and determine the role for FHWA.

Researchers have investigated a variety of visibility issues related to transportation, including efforts to explore retroreflectivity, pavement-marking signs, and legibility of fonts; however, much of this work has been tapering off in recent years. The workshop resulted in meaningful dialogue about necessary future research and FHWA’srole.

Much of the research conducted on visibility has focused on the energy sector and the vehicle side, not transportation infrastructure. The research has resulted in several advances in sight and cognition that move beyond existing processes and frameworks used in highway visibility.

The workshop included presentations from five experts, covering topics such as FHWA’s previous research, visibility needs for vehicles of the future, roadway lighting limitations, eye-movement analysis, and retroreflectivity. After the presentations, the participants discussed current issues and identified gaps where further research could help move the field forward. Participants also considered exploratory research needed and how that research could be coordinated across disciplines.

A key takeaway from the workshop is that current visibility standards and guidelines for new technologies and the roadway implications of these technologies need to be revisited and reestablished. Another takeaway is that driver, pedestrian, and night simulation studies are needed to provide insight into a driver’s decisionmaking processes when visibility is low.

The report is available to download at