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Public Roads - November/December 2015

November/December 2015
Issue No:
Vol. 79 No. 3
Publication Number:
Table of Contents

Communication Product Updates

Communication Product Updates

by AUTHOR 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

Synthesis of Geosynthetic Reinforced Soil (GRS) Design Topics (Report)

Publication Number: FHWA-HRT-14-094

dep8_0Geosynthetic reinforced soil (GRS) technology consists of closely spaced layers of geosynthetic reinforcement and compacted granular fill material, used for a variety of earthwork applications. This report features six topics related to GRS design, as differentiated from mechanically stabilized earth design. Those topics include: (1) embedment length, (2) pullout check, (3) eccentricity, (4) lateral pressures, (5) the equation for soil geosynthetic capacity for bearing resistance and required reinforcement strength, and (6) long-term geosynthetic reduction factors. The report includes a research summary of each topic and provides discussions of pertinent issues related to current practice.

Embedment length refers to the length of reinforcement within the GRS mass. This synopsis includes a review of research and case histories on the use and performance of shorter reinforcement lengths.

The report discusses the implications of performing or not performing the design check for reinforcement pullout of a GRS structure. The pullout check is intended to ensure adequate reinforcement length to prevent a structure from failing due to load and pressure--causing the reinforcements to pull out of the soil, for example. The report covers the history, research, and performance data and their impact on the requirement for pullout for GRS design.

Eccentricity, also referred to as overturning, is a principal component in the design of certain concrete walls with relatively rigid footings. This report details the purpose and theoretical justification of eliminating the need to limit eccentricity considering the flexible nature of GRS.

Lateral pressures against a GRS face include soil compaction, the movement of the facing, and the spacing of the reinforcement. This synopsis covers the effects of compaction-induced stresses on the internal lateral stress of both unreinforced and reinforced soil. Researchers discuss measured thrusts against facing elements and compare methods for estimating this thrust with measured values.

The researchers also discuss how to use the soil geosynthetic equation to estimate the bearing resistance and required reinforcement strength of GRS walls and abutments. The equation uses an empirical W-term, or factor, that accounts for the reinforcement spacing and maximum aggregate size in a structure.

Lastly, the report examines long-term strength reduction factors and discusses differences between current practices for GRS and mechanically stabilized earth design. This topic includes a summary of the research and the associated theory behind the reduction factors of creep, degradation (or durability), and installation damage.

The report is available to download at Printed copies are available from the PDC.

Factors Influencing Operating Speeds and Safety on Rural and Suburban Roads (Report)

Publication Number: FHWA-HRT-15-030

Cover of the report Factors Influencing Operating Speeds and Safety on Rural and Suburban Roads.Drivers who exceed the speed limit or drive too fast for the road conditions are involved in nearly one-third of all fatal crashes. More than 10,000 people are killed in speed-related crashes each year. This report documents the component factors affecting speed and safety on rural and suburban roadways that are not limited access. The report also describes treatments that have the potential to reduce speed-related crashes.

The first phase of the study included a review of literature on design features and current practices associated with safer operating speeds and identification of treatments for field evaluations. The second phase evaluated treatments to determine their effectiveness in reducing speeds on two-lane horizontal curves in rural and suburban areas. The study assessed high-friction surface treatments, optical speed bars, and various combinations of lane and shoulder widths.

Researchers evaluated high-friction surface treatments at four sites compared with three control sites in West Virginia. The analysis confirmed that friction increased. However, speed and encroachment analyses revealed no consistent differences between the periods before and after applying the treatment. These inconclusive findings mean that further research is needed. Currently, FHWA is conducting an observational before-and-after safety evaluation under a separate project, “Evaluations of Low Cost Safety Improvements Pooled Fund Study.”

Optical speed bars are short transverse stripes along the edges of a lane spaced at gradually decreasing distances. They are intended to increase drivers’ awareness of speed and so cause them to reduce speed. Researchers implemented and evaluated the stripes at eight sites in Alabama, four in Arizona, and seven in Massachusetts. They tested two different designs and found inconsistent speed reductions at all test sites, concluding that the designs used in this study were unsuccessful in reducing vehicle speeds.

The researchers also estimated the safety effects of combinations of lane width and shoulder width on rural two-lane, two-way road segments in Illinois and Minnesota. Although they found it difficult to distinguish differences in the performance of an 11-foot (3.4-meter)-wide lane compared to that of a 12-foot (3.7-meter)-wide lane, increasing the shoulder width did decrease the expected total number of crashes (all types and severities). When they studied the interaction of lane width and shoulder width, the researchers found that shoulder width has the greatest effect on safety when the lane width is 10 feet (3 meters). The effect of shoulder width lessens as lane width increases.

This report is available to download at

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

Publication Number: FHWA-HRT-15-034

dep10Open-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 quality assurance testing, and have the advantages of very low fine particle content, lighter unit weight, free-draining characteristics, and low frost heave potential.

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 these materials 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. The researchers also investigated relationships between other important soil parameters and the friction angle, as well as the effect of different automated testing devices and methods to interpret the data.

The report presents strength characteristics of 16open-graded aggregates representative of commonly selected structural backfills. The test results indicate higher strengths for these materials than current default values typically assume 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

Foundation Characterization Program (FCP): TechBrief #1--Workshop Report on the Reuse of Bridge Foundations (TechBrief)

Publication Number: FHWA-HRT-14-072

dep11This technical brief presents the results of a workshop on the reuse of bridge foundations held in 2014 at the Transportation Research Board’s 93rd Annual Meeting. The workshop was part of ongoing efforts by FHWA to identify needs for research and development with respect to FHWA’s Foundation Characterization Program. The program includes the development and evaluation of methodologies for characterizing existing bridge foundations for the determination of unknown geometry, material properties, integrity, and load-carrying capacity.

Workshop participants identified reuse applications for bridge foundations, including replacing the superstructure of an existing bridge because of structural deterioration, widening an existing bridge, repurposing (for example, using an existing railroad bridge foundation for a pedestrian/bicycle path), and reuse with strengthened foundation and accelerated bridge construction methods.

The reuse of existing foundations faces many challenges. Evaluating an existing bridge foundation for reuse is a multidisciplinary task that calls for structural, hydraulic, geotechnical, and construction expertise. Workshop presenters recognized that the most important need for advancing the reuse of bridge foundations is development of FHWA best practices and technical guidance for evaluating existing foundations and mitigating the risks involved.

The technical brief is available to download at

Investigating Improvements to Pedestrian Crossings with an Emphasis on the Rectangular Rapid-Flashing Beacon (Report)

Publication Number: FHWA-HRT-15-043

dep12_4The rapid-flashing beacon has received national attention as a way to emphasize the presence of a pedestrian crossing the roadway, but practitioners have asked whether the shape of the beacon plays a role in its effectiveness. This report documents an FHWA project that investigated how characteristics of rapid-flashing beacons (shape, size, and brightness) affect the ability of drivers to detect people or objects along the roadway and the likelihood of drivers yielding to a pedestrian.

In the first phase of the project, researchers reviewed recent literature and crash data to identify trends in pedestrian safety and in the effectiveness of crossing treatments. They also conducted limited field observations at 10 crosswalks in 5 States as a source of ideas for evaluating crossings in the second phase of the project.

Research in the second phase included a closed-course study and an open-road study to determine what characteristics of rapid-flashing beacons affected drivers’ ability to detect people or objects, as well as drivers’ likelihood of yielding to a pedestrian. The closed-course study included 71 participants who drove the course and viewed 8beacon assemblies, 9 distractor signs, and up to 11 roadside objects. The open-road study involved both circular and rectangular beacons installed at 12 sites located in 4 cities: Austin and College Station, TX; Flagstaff, AZ; and Milwaukee, WI.

Although the researchers found a slight difference between the average percentage of drivers yielding to circular versus rectangular beacons (daytime: 67 percent and 59 percent, respectively; nighttime: 69 percent and 72 percent, respectively), the statistical evaluation determined that the shape of the beacon did not have a significant effect on drivers’ responses. However, the results showed that a driver is more than three times as likely to yield when a beacon is activated as when it is not activated. Other variables that had an impact on yielding included beacon intensity (for nighttime) and city (yielding was higher in Flagstaff, AZ, compared with the other cities included in study).

The document is available to download at