Enhanced guidelines, analysis tools, and decisionmaking approaches are driving the latest innovations available to transportation professionals.

 

 

Today’s transportation landscape is fraught with challenges spurred by limited budgets and competing demands. Revenue streams for infrastructure projects are less certain, while aging infrastructure requires repair or replacement as congestion grows in many urban areas. Design decisions have become increasingly complex, demanding analyses of many factors and potential outcomes.

Balancing competing demands is essential to providing a safe and reliable system for automobiles, freight carriers, pedestrians, bicyclists, and transit users while reducing environmental impacts and enhancing surrounding communities. To address these challenges and meet transportation goals for the future, State and local departments of transportation are embracing innovative approaches to guide decisionmaking and using emerging tools to achieve performance objectives.

By linking design and engineering decisions to explicit performance outcomes, agencies are striving to be more strategic in their allocation of resources. A quality design must satisfy the needs of users and balance cost, safety, and mobility with historical, cultural, and environmental impacts. Designers work with complex relationships between these competing interests, enabling flexibility and managing the related risks, such as designing intersections for freight mobility and pedestrian safety.

Recently, transportation agencies across the United States have been bridging knowledge gaps and enabling engineers to design with increased flexibility and to employ context-sensitive approaches with greater confidence and regularity.

What follows is a detailed look at the status of principles and practices affecting geometric design in the United States. The American Association of State Highway and Transportation Officials and the Federal Highway Administration presented these innovations in a detailed report at the Transportation Research Board’s International Symposium on HighwayGeometric Design in June 2015.

Practical and Performance-Based Practical Design

To address system performance, mobility, and safety needs, some State DOTs have moved toward a practical design process, while others are moving toward a performance-based practical design approach.

Practical design emphasizes a renewed focus on scoping projects to stay within the core purpose. The name, definition, and approach of practical design varies from State to State. However, the principle and goal are the same: exercising a greater level of discipline to eliminate nonessential design elements, resulting in lower project costs and improved value. This approach could enable transportation agencies to deliver a greater number of projects than was possible under their traditional project development mechanisms. Practical design enables agencies to look beyond individual projects and consider the overall system benefits during design decisionmaking.

Performance-based practical design helps agencies further refine their focus by incorporating performance goals into their decisionmaking. This helps ensure that agencies do not overemphasize short-term cost savings without a clear understanding of how such decisions could affect other objectives, such as safety and operational performance, pedestrian and bicycle connectivity, context sensitivity, life-cycle costs, long-range corridor goals, livability, and sustainability. For example, performance-based practical design can be as simple as modifying a traditional design approach to a “design up” approach, where transportation decisionmakers exercise engineering judgment to build up the improvements from existing conditions to meet both project and system objectives.

According to Robert Mooney, preconstruction team leader in FHWA’s Office of Infrastructure, performance-based practical design provides a flexible means to meet improvement objectives. “As we focus on improving system performance,” he says, “we are excited about using the flexibility and performance analysis associated with performance-based practical design to support transportation investments in project- and program-level decisions.”

Context-Sensitive Solutions

Context-sensitive solutions (CSS) represent another aspect of geometric design that can influence transportation processes, outcomes, and decisionmaking. FHWA describes CSS as a collaborative, interdisciplinary approach to the transportation planning and development process that involves all stakeholders in designing a facility that complements its physical setting and preserves scenic, aesthetic, and historic and environmental resources while maintaining safety and mobility. CSS help meet community and national goals of environmental sustainability, improve cost-effectiveness, and streamline the delivery of transportation programs.

Transportation agencies have made progress on institutionalizing CSS in their business processes, but a recent informal survey by FHWA revealed that more work is necessary. Nearly one-quarter of the States rated themselves as either in the “Initiating Progress” or “Early Implementation” stages of institutionalizing CSS.

 

 

To help fuel faster implementation, in 2015 the Institute of Transportation Engineers published the informational report Integration of Safety in the Project Development Process and Beyond: A Context-Sensitive Approach. The report aims to expand understanding of CSS principles and practices within the transportation community and to help agencies incorporate highway safety elements from a quantitative, analytical, and technical perspective.

SafetyPerformanceAnalysis

Several innovative research efforts are deploying a new generation of tools for analyzing highway safety. Efforts include the Highway Safety Manual (HSM) and supporting tools such as the Interactive Highway Safety Design Model (IHSDM) and road safety audits (RSAs). These tools are helping to advance State and local highway agencies’ ability to incorporate explicit, quantitative consideration of safety into their decisionmaking during planning and project development.

Safety has been consistently among the highest priorities for transportation agencies. But, until publication of the HSM in 2010, DOTs lacked a widely accepted tool to quantifiably predict the impact of infrastructure decisions on safety. The HSM includes predictive methods that transportation agencies can use to anticipate the safety performance of new facilities, to assess the safety performance of existing facilities, and to estimate the expected effectiveness of proposed improvements to existing facilities.

Two additional chapters of the HSM, released in 2014, add predictive methods for estimating the expected average crash frequencies on freeways and ramps. The predictive method for freeways includes evaluation of segments with and without speed change lanes. The predictive method for ramps includes evaluation of interchange components (ramps, collector-distributor roads, and ramp terminals).

AASHTO’s Standing Committee on Highway Traffic Safety aims to institutionalize the HSM and its associated analytical tools to support national goals to reduce highway fatalities and serious injuries. Current implementation of the HSM varies by State. Some lead States are executing comprehensive implementation plans, while others are moving toward implementation at a more moderate pace.

“Implementation of the HSM by States is reaching beyond the Highway Safety Improvement Program [HSIP], and they are using it in various phases of the overall transportation management process, including network screening, alternative analysis, exceptions to design policy, operations, and overall evaluation of effectiveness,” says Priscilla Tobias, State safety engineer for the Illinois Department of Transportation. “States can focus more on safety efforts and quantify the impacts of their safety decisions, which is critical as they work to leverage limited funding to have the greatest impact.”

FHWA’s IHSDM software offers a suite of analysis tools that engineers can use to evaluate the safety and operational effects of geometric design decisions on highways. IHSDM is a decision-support tool that provides estimates of a highway design’s expected safety and operational performance, and checks existing or proposed highway designs against relevant design policy values. Results generated using IHSDM can support better decisionmaking.

Another important tool for safety analysis is conducting RSAs during the project design phase. An RSA is a formal examination of the safety performance of an existing or future facility by an independent, interdisciplinary audit team. RSAs are a popular tool for evaluating the safety performance of inservice roads, and DOTs are now adapting them for use in evaluating design choices and opportunities for enhancing safety on projects in the design stage.

A 2004 publication of the National Cooperative Highway Research Program, Synthesis 336 Road Safety Audits: A Synthesis of Highway Practice, examined the practices and benefits of RSAs worldwide. The use of RSAs across the country has increased with all 50 States plus the District of Columbia and Puerto Rico having piloted an RSA and 16 States having established RSA programs by 2014.

SHRP2 Safety Data

An important factor for transportation design is the availability of safety data. A project supported by FHWA and AASHTO enables State transportation agencies and their research partners to use data developed through the second Strategic Highway Research Program (SHRP2) to improve methods for reducing crashes and enhancing highway safety. The Implementation Assistance Program is making datasets available to State DOTs to identify crash causation factors and to develop effective countermeasures, such as improved road designs, that will address their common safety concerns.

The SHRP2 safety data include two large databases: a Naturalistic Driving Study database and a Roadway Information Database. The data from the Naturalistic Driving Study database provide a wealth of information regarding driving behavior, and the Roadway Information Database houses data on roadway elements and conditions. These two databases are linked in order to associate driver behavior with actual roadway geometry and driving conditions.

Integrating Human Factors in Design

Knowledge of human factors is a critical component in designing safe and efficient roads. Human factors pertain to the capabilities and limitations of human beings as vehicle drivers, bicyclists, and pedestrians. Knowing how certain user groups are likely to respond to given conditions can help designers reduce the risk of user error, or at least minimize the consequences when an error occurs.

The NCHRP Report 600 Human Factors Guidelines for Road Systems, Second Edition, released in 2012, focuses on providing specific, actionable design guidance, supported by a discussion and review of key research and analyses. The guidelines help designers more effectively accommodate roadway users’ capabilities and limitations in the design and operation of highway facilities.

In addition, demographic trends in the United States indicate that by 2030, one-fifth of road users will be age 65 or older. This means that a steadily increasing proportion of road users will experience declining vision; slowed decisionmaking and reaction times; exaggerated difficulty when dividing attention between traffic demands and other important sources of information; and reductions in strength, flexibility, and general fitness. In a proactive response to the pending increase in aging road users, FHWA released the updated Handbook for Designing Roadways for the Aging Population (FHWA-SA-14-015) in 2014. The handbook provides practitioners with a practical source that links the performance of aging road users to highway design, operations, and traffic engineering features.

FHWA also sponsored research that investigated driver expectations and signage at complex interchanges. The results of Driver Expectations When Navigating Complex Interchanges (FHWA-HRT-13-048) and Simulator Study of Signs for a Complex Interchange and Complex Interchange Spreadsheet Tool (FHWA-HRT-13-047) indicate that complex interchanges pose significant challenges to most drivers, and that many of these problems arise from basic human factors issues related to various aspects of interchanges. For example, when a guide sign displays information that is unclear or is located too close to a decision point, drivers may miss their desired path or make risky maneuvers. FHWA is continuing research efforts on complex interchanges with the goal of developing useful design guidance related to perceptual elements of guide signs.

 

 

“Integrating human factors methods and techniques into research and design efforts allows researchers and roadway designers to better match driver expectations and behavior with signing and roadway geometry characteristics,” says Brian Philips, senior research psychologist with FHWA’s Human Factors Team in the Office of Safety Research and Development. “This facilitates better decisionmaking [for] all roadway users, promotes safer driving, and decreases the risk of crashes.”

Accessible Transportation

Another special consideration for geometric design is making transportation accessible and safe for all users. The Americans with Disabilities Act influences design decisions with its requirements to assure equal access to government programs (such as transportation) and to public places for people with physical or mental disabilities.

The current design standards for accessibility are the Americans with Disabilities Act Accessibility Guidelines; however, these standards are more applicable to building and site construction than to public right-of-way. Therefore, transportation agencies have been left to translate these standards, where applicable, to highways and streets and make judgments on issues not fully addressed by the standards.

The U.S. Architectural and Transportation Barriers Compliance Board (also known as the Access Board) is finalizing its Guidelines for Pedestrian Facilities in the Public Right-of-Way with its release anticipated this year. With these guidelines, considering accessibility early in the development of design alternatives will be more imperative than ever. For example, explicit guidelines for the cross-slope and grade of crosswalks will influence intersection grading and storm drainage.

When designers leave consideration of accessible pedestrian facilities until later in the design process, they may severely limit their opportunities--and inflate their costs--to develop a design that provides optimum functionality for all users.

 

 

Designing for Pedestrians And Bicyclists

Integrating features that respond to the needs of all pedestrians and bicyclists into street and highway infrastructure is another competing demand for transportation designers. Today’s transportation systems are placing increased emphasis on pedestrian and bicycle networks that are safe, comfortable, and convenient for people of all ages and abilities.

In 2015, Secretary of Transportation Anthony Foxx challenged mayors and other elected city officials across the country to “take significant action to improve safety for bicycle riders and pedestrians of all ages and abilities.” Key elements of this challenge include using designs that are appropriate to the context of the street and its uses, and taking advantage of opportunities to create and complete pedestrian and bicycle networks.

One of the challenge’s recommendations is to gather and track data on walking and bicycling. Communities that routinely collect such data are better positioned to tracktrends and prioritize investments.

FHWA also released updates to the Pedestrian Safety Guide and Countermeasure Selection System (PEDSAFE) in 2013 and the Bicycle Safety Guide and Countermeasure Selection System (BIKESAFE) in 2014. The guides assist transportation planners and engineers in selecting countermeasures to improve safety for these roadway users. For more information, visit www.pedbikesafe.org.

Another available resource is the 4th edition of AASHTO’s Guide for the Development of Bicycle Facilities. This guide includes information on the dimensions and operational characteristics of a range of facility designs for bicycles, recommendations for selecting the type of bikeway based on the characteristics of the roadway, and design details for a variety of roadway configurations that accommodate bicycle travel. The guide is a comprehensive treatment of the planning, design, safety, and operational considerations for bicycle facilities. To address some innovative techniques for onstreet bikeways not included in this guide, such as two-stage left turn boxes and bicycle signals, the National Association of City Transportation Officials released the Urban Bikeway Design Guide at http://nacto.org/publication/urban-bikeway-design-guide.

Several municipalities already are using separated bike lanes--sometimes referred to as cycle tracks--which created a need for guidance on designing these types of facilities. In response, FHWA released the Separated Bike Lane Planning and Design Guide (FHWA-HEP-15-025) in 2015. These guidelines address the benefits and challenges with the various design treatments and provide designers with background information on what is currently known about the safety of these designs. For more on separted bike lanes, see “Let’s Ride!” in the May/June 2015 issue of Public Roads.

Considerations for Managed Lanes

Facilities with high-occupancy toll (HOT) and high-occupancy vehicle (HOV) lanes have a number of features that require special attention with respect to geometric design. HOT/HOV lanes are examples of “managed lanes,” which involve a range of strategies and techniques to control the usage of freeway lanes to improve the efficiency of traffic flow. These strategies and techniques may be permanent or vary as conditions change. Transportation agencies use managed lanes to reduce congestion and travel time and improve trip reliability, while avoiding the costs and impacts associated with constructing additional lanes.

 

 

The HOT/HOV lanes are usually the left-most lanes on the freeway and typically operate at higher travel speeds than the adjacent general purpose lanes. The differential in speeds between adjacent lanes and the HOT/HOV lanes presents a potential crash risk. Roadway designers manage this risk by providing a buffer between the lanes and controlling ingress and egress from the HOT/HOV lanes. In zones where vehicles can change lanes between the general purpose lanes and the HOT/HOV lanes, the design needs to consider the location of the interchanges and required lane changes for entering or exiting the freeway. In many locations, designs incorporate direct access ramps between cross streets and the HOT/HOV lanes. At freeway-to-freeway interchanges, additional ramps enable motorists in the HOT/HOV facility to move directly to the HOT/HOV lanes of the other facility without traversing the general purpose lanes.

Where congestion is high and space to add new lanes is limited, shoulder running isanother managed--lane strategy that is becoming more prevalent. With shoulder running, DOTs use the existing shoulder as another travel lane, either for set periods when congestion is usually high, or they can designate use of the shoulder dynamically in response to conditions. Because shoulders serve many purposes--clear roadside, breakdown lane, enforcement activities, maintenance activities, and drainage--the need for additional lane capacity must be balanced with these other needs. Shoulder running strategies necessitate careful consideration of lane configurations at interchanges, reduced clear zone on the roadside, speed, emergency pulloffs, and enforcement.

For more information, visit www.ops.fhwa.dot.gov/atdm/approaches/atm.htm.

Emerging Technical Areas

FHWA is encouraging greater flexibility in geometric design through its Every Day Counts (EDC) initiative. EDC aims to accelerate the deployment of innovative practices and technologies to deliver safer, more efficient projects with shorter delivery times. Several EDC initiatives relate to improving safety through better design: alternative intersections and interchanges, road diets, high-friction surface treatments, the Safety Edge^SM, and three-dimensional (3–D) modeling. Each is briefly described below, and more information is available at www.fhwa.dot.gov/innovation/everydaycounts.

“Innovations are being tried and proven locally throughout the Nation,” says Thomas Harman, director of FHWA’s Center for Accelerating Innovation. “Every Day Counts takes those market-ready breakthroughs and fosters widespread adoption, so the transportation community and the traveling public benefit sooner.”

Innovative Intersection and Interchange Geometrics

At many highway junctions, traffic and safety problems are more complicated than ever because of increased congestion. Conventional intersection designs might be insufficient to mitigate some transportation challenges. Increasingly, designers are investigating and implementing innovative intersection treatments.

 

 

EDC is advancing select alternative intersection and interchange designs with substantial and proven benefits compared to conventional designs. Among those designs are displaced left-turn intersections, variations on U-turn intersections, diverging diamond interchanges, and modern roundabouts.

 

 

In the Alternative Intersections/Interchanges: Informational Report (FHWA-HRT-09-060), published in 2009, FHWA presents information on these intersection and interchange designs. Since then, much additional experience and information has become available. Through EDC, FHWA is updating information on the geometric design features, operational and safety issues, access management issues, costs and construction sequencing, and applicability of several innovative intersection and interchange designs. Four new informational guides provide details on the restricted crossing U-turn, median U-turn, displaced left-turn intersections, and diverging diamond interchanges. The guides are available at http://safety.fhwa.dot.gov/intersection under Innovative Intersections.

Road Diets

Roadway reconfigurations, also known as road diets, are another safety innovation promoted through EDC. A typical road diet converts an existing four-lane, undivided roadway to three lanes: two through lanes and a center, two-way left-turn lane. However, a road diet may be applied to streets with more than four lanes, and may simply narrow lanes rather than reduce their number. Although they may take many forms, the key feature of a road diet is that it reallocates space for other uses, such as turn lanes, bus lanes, pedestrian refuge islands, bike lanes, sidewalks, bus shelters, parking, or landscaping.

Road diets offer several benefits including reduced crashes, traffic calming, enhanced access for all road users, and a complete streets environment to accommodate a variety of transportation modes. Transportation agencies often can implement a road diet at relatively low cost by incorporating it into a planned resurfacing project with adjusted signing and marking.

FHWA recently produced a Road Diet Informational Guide (FHWA-SA-14-028) to help communities understand the safety and operational benefits and determine if road diets could be helpful on their roads. A complementary publication, Road Diet Case Studies (FHWA-SA-15-052), lists real-world examples of road diets already implemented across the country.

High-Friction Surface Treatments

High-friction surface treatments (HFST) are site-specific applications of very high-quality, durable aggregates using a polymer binder that restores and maintains pavement friction. Maintaining the appropriate amount of pavement friction is critical for safe driving, especially at horizontal curves and intersections. Vehicles traversing horizontal curves require a greater side force friction, and vehicles at intersections require greater longitudinal force friction.

Horizontal curves make up only 5percent of U.S. highway miles, but more than 25percent of highway fatalities occur at or near horizontal curves each year. Although some of the factors contributing to these crashes include excessive vehicle speed or distracted driving and driver error, at some locations, the deterioration of pavement surface friction may also be a contributing factor. Variable friction creates the need for pavement surface improvements, particularly for friction, at certain locations to increase safety. Although the largest numbers of problem locations are likely on local and collector systems, these treatments could prove beneficial at high-volume intersections, interchange ramps, and selected segments of interstate alignments.

Safety Edge

Roadway departures account for more than half of all fatal crashes in the United States. The dropoff of the pavement edge on roadways is a contributing cause of many of these crashes. The Safety Edge, another EDC innovation, is a low-cost technology that enables drivers who drift off highways to return to the pavement safely. Simply shaping the edge of the pavement to 30 degrees mitigates the safety problem of vertical dropoffs. The angled Safety Edge provides a durable transition on which vehicles can return to the paved road smoothly and easily, even at relatively high speeds.

3–D Engineered Models

Three-dimensional (3–D) engineered models are used widely by the highway community to more effectively connect a project’s design and construction phases. These models and the as-found, digital geospatial data that support them also can be applied to other phases in the project delivery cycle to positively affect safety, project scheduling, project costs, contracting, maintenance, and asset management. Today, construction firms of all sizes are investing in automated machine guidance for their equipment. The equipment has advanced to a point that even precision work, such as paving, is possible without traditional survey and staking.

 

 

To take full advantage of their investment, the contracting industry is working with State DOTs to expedite the transition to electronic files. Many States are now using design software to export 3–D terrain files, which they can transfer directly to contractors’ equipment. The direct transfer reduces the chance of survey errors and typos that can occur when contractors have to copy the data out of paper plans. As the contract documents move away from traditional paper formats and toward electronic transfer of data, States are placing an increased emphasis on accuracy of the models, consistency of data formats, the integrity of data during usage, file security, digital signatures, and related issues.

The 3–D models that States are using also can provide valuable insights that can help designers evaluate complex features such as intersection alignment, sight distance from a driver’s view, and conflicts or “clashes” that may occur during construction. This type of modeling enables designers to view future projects in a simulated environment, with much less work than was previously required. They also can evaluate nontraditional designs in much greater detail. With the success of 3–D design, some States are moving to 4–D and 5–D design, which incorporate schedule (4–D) and cost (5–D), to take even greater advantage of the model as a project management tool.

In addition to 3–D modeling, agencies are using data collection tools that can create virtual models of existing features with greater accuracy and precision. For example, light detection and ranging (LiDAR) uses laser pulses to measure distance, providing significantly more data points with less disruption to traffic. As LiDAR usage increases and the quality of mobile data collection improves, designers have the opportunity to develop high-resolution models with even more information in greater detail and accuracy than was possible with traditional survey and photogrammetric practices. Using these models with historical design details and crash data, designers can identify locations that present an increased crash risk and make cost-effective asset management decisions.

 

 

What’s Next: V2I and V2VTechnologies

As communication and mapping technologies become more sophisticated, the U.S. Department of Transportation, automobile manufacturers, and others are investigating opportunities and developing plans to use these technologies to improve highway safety and mobility.

For example, vehicle-to-infrastructure (V2I) and vehicle-to-vehicle (V2V) communications enable drivers to receive enhanced information based on their speed and location with respect to roadway features and other travelers, including pedestrians and bicyclists. Analyses of V2I applications reveal an opportunity to eliminate 59percent of single-vehicle crashes and 29percent of multivehicle crashes. This significant reduction could be possible without making physical changes to roadway geometry.

The potential for development of more V2I and V2V applications and the implementation of automated driving tasks raise questions about the traditional way of thinking about geometric design. The implications of these technologies will play a significant role in the evolution of decisions regarding geometric design.

Looking to the Future

Although safety continues to be among the highest priorities in highway design decisions, increasingly State DOTs are being called upon to achieve safety goals in balance with other priorities. Many States and localities are embracing the national strategy Toward Zero Deaths to emphasize the need to continue making improvements to prevent traffic fatalities. Meanwhile, the U.S. Congress has enacted legislation requiring the evolution toward performance-based administration of the transportation system, with agencies setting specific targets to realize improvements in planning, safety, highway conditions, and transit.

Geometric design is the means by which agencies achieve many transportation goals and uphold values in communities. Agencies across the Nation are employing new approaches to design decisionmaking to assure the best use of their resources in achieving their transportation performance goals. The newer and evolving tools and knowledge are changing how designers perceive their roles.

“With improved knowledge about the needs of all road users and emerging means of predicting and modeling the effects of our decisions, designers can more fully understand the performance implications of complex design decisions,” says Mike Griffith, director of FHWA’s Office of Safety Technologies. “These exciting changes provide greater opportunities to incorporate innovation into design decisionmaking and assure that these choices truly contribute to a successful transportation system for the future.”

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Brooke Struve, P.E., is a safety and geometric design engineer with the FHWA Resource Center, providing technical assistance and training on geometric design flexibility and designing for safety of all users. She graduated from Brigham Young University with a B.S. in civil engineering.

Mark Doctor, P.E., is a safety and geometric design engineer with the FHWA Resource Center, where he provides technical services and training on deploying innovative and flexible design and safety practices at a national level. Doctor received a bachelor’s degree in civil engineering from Clemson University and a master’s degree in transportation engineering from the University of Florida.

Deanna Maifield, P.E., is the assistant design engineer for the Iowa Department of Transportation and a member of the AASHTO Technical Committee on Geometric Design. Maifield is a graduate of Iowa State University with a bachelor’s degree in civil engineering.

Shyuan-Ren (Clayton) Chen, Ph.D., P.E., PTOE, is the roadway team leader in the Office of Safety Research and Development with FHWA. He leads FHWA’s research, development, and technology efforts for geometric design, safety analytical tools, roadside safety, intersection and interchange safety, speed management, and intelligent transportation system/connected-vehicle safety applications. Chen holds a Ph.D. from the University of Connecticut and a master’s degree from the University of Texas at Arlington, both in civil engineering.

For more information, see the full report at www.fhwa.dot.gov/design/standards/county.pdf.