U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: FHWA-HRT-10-006 Date: September/October 2010|
Publication Number: FHWA-HRT-10-006
Issue No: Vol. 74 No. 2
Date: September/October 2010
Recent innovations and improved guidance in geometric design are helping agencies overcome some of the most urgent transportation challenges.
|This roundabout in North Bend, WA, is an example of highway engineers doing more with less: They selected a geometric design that reduces pedestrian-related conflicts, crash severities, vehicle idling, and maintenance costs.|
In recent years, scarce financial resources, limited rights-of-way, and continuing concerns about the environmental footprint of highway projects have compelled transportation officials to do more with less when building and maintaining the Nation's roadways. Despite these hurdles, the Federal Highway Administration (FHWA) and its State and local partners face mounting demand for greater capacity, safety, and efficiency.
Experts predict the situation will become even more challenging in the years ahead. Some estimate that vehicle-miles traveled (VMT) will increase by 50 percent from 2005 to 2030, while freeway lane-miles will grow relatively slowly. A freeway that today carries 20,000 vehicles per day will carry 30,000 in two decades, within essentially the same footprint. In short, the existing system must accommodate the overall increase in VMT, but with little added capacity except at the most troublesome bottlenecks.
However, recent innovations and improved guidance on geometric design are helping address this urgent transportation challenge. As defined in the Institute of Transportation Engineers' Urban Street Geometric Design Handbook, "geometric design" refers to the dimensions and arrangements of the visible features of a street, including pavement width, horizontal and vertical alignment, slope, and channelization, that affect the roadway's operations, safety, and capacity.
Highway design engineers are taking advantage of new tools, technologies, and practices in geometric design to improve the quality and efficiency of the transportation system. Among them are design flexibility, performance- and risk-based design approaches, human factors research, road safety audits (RSAs), improved work zones, managed lanes, and design visualization. Together, these innovations are helping State departments of transportation (DOTs) wring the most value from each dollar and making a difference in safety and mobility.
One recent development in geometric design is the shift away from the traditional standards-based approach to a process known as "flexible design." Engineers apply design practices based on long-established criteria and established guidance found in design manuals when using the standards-based approach. First, they identify a transportation problem, develop potential solutions in the form of project alternatives, and then explain and defend the alternatives at public meetings.
On the other hand, flexible design considers the interests of a wide variety of highway users early in the project development process. Through creativity and collaboration with the public, engineers balance cost, safety, and mobility with historical, community, cultural, and environmental impacts.
"Flexible design involves thinking beyond just roadway engineering," says George Merritt, a safety engineer with FHWA's Resource Center. "It's about applying with greater confidence and regularity the approach known as context sensitive solutions [CSS], in which transportation facilities are integrated seamlessly into their environments while meeting safety and mobility goals."
Flexible design is about making well-informed choices. Simple application of the highest or lowest value within a range of design values without explicit consideration of context does not always lead to the most informed choices that best meet a project's objectives. In addition to collecting traffic data, engineers using flexible design have to obtain information about a project's unique contextual characteristics and what the public considers desirable for the corridor.
Flexible design and CSS contribute to creating livable communities where neighborhoods are preserved; farms, forests, and other green spaces are protected; parents spend less time in traffic and more time with their children, spouses, and neighbors; and communities have access to multiple modes of transportation. In fact, U.S. Transportation Secretary Ray LaHood recently identified community livability as a top priority, entering the U.S. Department of Transportation into a partnership with the U.S. Department of Housing and Urban Development and the U.S. Environmental Protection Agency to coordinate Federal investments in transportation, housing, and environmental protection. The partnership aims to help families in rural, suburban, and urban areas alike access affordable housing and more transportation options, while simultaneously protecting the environment and helping address the challenges of climate change.
Design manuals, such as the American Association of State Highway and Transportation Officials' (AASHTO) A Guide for Achieving Flexibility in Highway Design, stress the need for knowledgeable, skilled highway engineers to execute successful context-sensitive projects. State DOTs are updating their manuals to provide designers and decisionmakers with frameworks for incorporating flexible design into transportation improvement projects. (For more information, see "Risking Success Through Flexible Design" in the January/February 2010 issue of Public Roads.)
|This drawing depicts a double crossover diamond (DCD) interchange, an innovative roadway design that has emerged through application of flexible design. At a conventional diamond interchange, drivers turn left across the path of opposing through traffic, but flipping the traffic streams in a DCD interchange removes the conflict between the left turn from the major road and the opposing through movement.|
Flexible design involves highway engineers and designers working to allow flexibility in roadway design while managing the related risks -- uncertainties that can have positive or negative impacts on a project. This approach considers the safety and performance tradeoffs associated with using minimum or alternative geometric criteria and standards. An example of the risk-based design approach, as applied to rural and high-speed facilities, according to AASHTO's Guidelines for Geometric Design of Very Low-Volume Local Roads, is a "proposed action that is expected to result in no more than one additional traffic crash . . . per mile of roadway every 6 to 9 years."
The fundamental question in this performance- and risk-based approach is this: Can the purpose of a project be addressed through a minimal design that achieves the desired level of safety and operational performance, but at a level of risk an agency is willing to accept?
"This back-to-basics approach challenges designers to be creative," says Jeffrey Shaw, a highway engineer with FHWA's Office of Safety Design. "It allows us to engineer a solution that is consistent with the principles of traditional design criteria, but also allows maximum flexibility to consider the context and interaction of various design decisions, the associated impacts to safety and operations, and the cost effectiveness and risk associated with those decisions."
Practical design identifies the risks related to a design and calls for fully understanding, evaluating, and mitigating those risks. No facility will ever be designed risk-free, but this approach encourages designers to weigh and manage the risks inherent in operating a road network rather than taking a strictly standards-based approach. Each DOT can decide which risks, if accepted, can be turned into rewards, such as reduced costs and time, minimized environmental impact, reduced right-of-way required, and preserved community assets.
Geometric design decisions increasingly will be based on improving performance while still meeting traditional criteria. In fact, many State DOTs are basing highway designs on achieving performance-based goals. For example, the Virginia Department of Transportation developed an online performance reporting system for projects and programs. The department's "Dashboard" (http://dashboard.virginiadot.org) displays information on highway performance, safety, pavement conditions, and financing, along with information on the agency's project completion rate, management, and customer satisfaction.
The Strategic Highway Research Program 2 (SHRP 2) Report S2-CO2-RR, Performance Measurement Framework for Highway Capacity Decision Making, describes specific performance factors considered in design, such as mobility, reliability, accessibility, and safety.
Another aspect of performance- and risk-based design deals with whether a proposed design will meet the needs of its users. Designers are employing new techniques and tools to provide roads that respond to the needs of both motorized and nonmotorized traffic while ensuring safety for all. Notable among these approaches is use of human factors guidelines and RSAs, consideration of nonmotorized vehicles, improvements to work zones, use of managed lanes, and use of design visualization.
|FHWA's Highway Driving Simulator, shown here, enables engineers to develop a design and then have potential end users test drive the proposed facility early in the design process.|
Doing more with less requires fully optimizing geometric designs to facilitate and encourage safe driver behaviors and reduce errors in judgment. The objective is to decrease both the likelihood and severity of adverse effects of poor driving decisions. Human factors -- the capabilities and limitations of people as vehicle drivers, bicyclists, and pedestrians -- are important considerations in design decisionmaking.
Highway agencies and design professionals increasingly are applying the human factors approach based on improved understanding of user behavior in relation to geometric design elements. Armed with knowledge about how a given type of user is likely to respond to an element, road designers can make more informed decisions about how to reduce the likelihood of user error, or at least minimize the consequences of an error.
FHWA's Turner-Fairbank Highway Research Center (TFHRC) and the Missouri Department of Transportation (MoDOT) demonstrated the human factors approach when they tested the design of a relatively new type of interchange, a double crossover diamond (DCD), in Kansas City, MO. TFHRC's Human Centered Systems Research team helped MoDOT build a simulation of the DCD, using three-dimensional (3-D) and four-dimensional (4-D) visualizations, in FHWA's Highway Driving Simulator.
The team recruited more than 70 volunteers to participate in test drives through the simulated DCD. The exercise exposed sight distance conditions at the interchange that otherwise might not have been noticed. The simulation also revealed unintended driver behaviors that resulted from a previous attempt to mitigate sight distance problems at traffic signals due to a curved roadway approach to the DCD. The exercise showed that other types of driver errors were no more likely to occur with the DCD than with a conventional interchange.
Enabling MoDOT's engineers to test their own designs, and then inviting potential end users to test drive the proposed interchange early in the design process, gave designers valuable information to improve safety and operational efficiency before the facility was built.
Another important design development is an RSA, which is a formal examination, by an independent, interdisciplinary team of professionals, of the safety performance of a road or intersection. FHWA considers RSAs to be a best practice for enhancing project safety. RSAs have become a popular tool for evaluating the safety performance of inservice roads, and the safety community often uses them to review a road in response to public inquiries, political interest, or increased crashes.
|Flexible design considers the needs and interests of a wide variety of users, such as these bicyclists and pedestrians at a roundabout at the University of California, Davis.|
Agencies also use RSAs proactively, during the design phase, to identify safety problems before they develop and to improve the features of a proposed geometric design. Over the past 2 years, more DOTs have begun conducting RSAs during the design stages of high-profile or high-cost projects, such as major corridor projects or freeway interchanges in metropolitan areas.
Coinciding with that trend, FHWA is working with the value engineering community to explore how to coordinate RSAs and value engineering studies to ensure that the value analysis process appropriately considers safety. Value engineering is a review or analysis of a proposed design to identify and recommend alternatives that reduce life-cycle costs and add value to a facility.
At the 2009 AASHTO Value Engineering Conference, FHWA sponsored a half-day workshop to explore what an integrated RSA-value engineering effort would look like. Based on feedback from the conference, FHWA plans to begin developing guidelines and case studies on successfully integrating the two practices.
Awareness of pedestrian and bicyclist safety and design issues has increased greatly over the past decade, leading to infrastructure improvements that benefit those users. Increasingly, the public expects bike lanes, sidewalks, recreational paths, and safer crosswalks to be part of any significant roadway construction project.
Design policies and practices also have changed, thanks in part to safety research and the efforts of organizations such as Walkable Communities (www.walkable.org) and the National Complete Streets Coalition (www.completestreets.org). Advocates of livable communities are helping advance pedestrian- and bicycle-friendly facilities by encouraging State and local transportation agencies to adopt statutes, ordinances, and policies requiring designers to provide features that meet the needs of all users. (For more information, see "Complete Streets" in the July/August 2010 issue of Public Roads.) An Institute of Transportation Engineers' report, Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities (www.contextsensitivesolutions.org/content/reading/ite036_css/), provides guidance on designing major urban streets "to support walkable and bikeable communities, compact development, and mixed land uses."
As the Nation puts more emphasis on maintaining and rehabilitating the existing transportation system, construction zones have become commonplace. As a result, designing safe and efficient work zones has become a high priority within the geometric design discipline.
The combination of more traffic and congestion, growing concerns about safety, and public frustration with more work zones led FHWA to publish the Work Zone Safety and Mobility Rule in September 2004. The regulation facilitates "consideration of the broader safety and mobility impacts of work zones in a more coordinated and comprehensive manner across project development stages."
|Because of potential impacts on safety and mobility, FHWA and State DOTs have made designing safe and efficient work zones, such as this one at a roundabout in Reno, NV, a high priority in the geometric design discipline.|
Work zone impacts often extend beyond the physical construction or maintenance activity. Congestion can occur especially on nearby roadways. The new regulation requires designers and managers to expand their thinking from work zone traffic control to transportation management in and around work zones.
The concept of work zone transportation management extends beyond accommodating traffic through the physical work zone to emphasizing the safety of the traveling public and workers. The concept also focuses on regional mobility and operations. One critical component is an outreach campaign to communicate to the public, road users, area residents, businesses, emergency responders, and other public entities about a road construction project and how it will affect them.
The work zone rule has helped integrate consideration of work zone impacts throughout the entire project development and delivery process. Designers now work with numerous stakeholders to outline construction sequencing in the design and develop comprehensive transportation management plans. These plans include detailed specifications for temporary traffic control, traffic operations, and public information. Designers now proactively study details that often were left for construction contractors and maintenance forces to handle.
Another way transportation agencies are doing more with less is through managed lanes. FHWA defines managed lanes as facilities, or a set of lanes, that transportation agencies manage in real time in response to changing conditions. Toll lanes and high-occupancy vehicle (HOV) lanes are examples. Managed lanes also can include part-time use of shoulders during peak hours for additional capacity.
The distinction between managed lanes and other forms of freeway management is the concept of active management, which entails proactively applying new strategies or modifying existing ones in response to demand to optimize capacity on a facility. Active management involves defining the operating objectives for managed lanes and the kinds of actions the agency will take once traffic volumes reach predefined performance thresholds. For example, in response to growing traffic demand on the managed lanes, an agency could decide to raise toll rates or increase the occupancy rate to reduce demand and maintain congestion-free travel.
Lane management includes pricing, when an agency applies a toll to a lane during certain periods to manage demand and maintain a performance threshold. Another application is vehicle eligibility, when an agency manages lanes by allowing or barring vehicles according to the number of their occupants. In access control, an agency limits vehicle access over long stretches of a facility, minimizing turbulence in vehicle flow.
The premise of active management is one of the distinguishing features of managed lanes. Transportation agencies can define the performance objectives and kinds of actions taken to maintain the thresholds. Some examples include raising the occupancy requirement to use an HOV lane to maintain operating speeds of 50 miles (80 kilometers) per hour, or closing an on-ramp to express lanes during peak periods so they can operate at a threshold of 1,500 vehicles per hour per lane.
HOV lanes are an early example of managed lanes in the United States. Design guidelines for HOV lanes have evolved based on various operational strategies and practices. AASHTO's Guide for High-Occupancy Vehicle (HOV) Facilities suggests methods and designs for dedicated facilities and preferential treatments to encourage greater use of HOVs on existing transportation systems. AASHTO has excerpted portions of the guide from the previous edition and supplemented them with material from the National Cooperative Highway Research Program's (NCHRP) Report 414 HOV Systems Manual and from the Texas Transportation Institute.
Adapting existing HOV-only lanes into managed lanes is an evolving concept that gives transportation agencies more flexibility to accommodate a wider range of users, including single-occupant vehicles under certain conditions. The Value Pricing Pilot Program, initiated under the Intermodal Surface Transportation Efficiency Act of 1991 and most recently renewed in the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users, introduced value pricing, or congestion pricing. The program enables agencies to work with FHWA to manage congestion by employing road pricing strategies, including charging motorists a toll for travel during the most congested times or offering a discount for offpeak travel. The program also ushered in the concept of high-occupancy toll (HOT) lanes as an operational strategy.
|This photo shows an example of HOV managed lanes in Gwinnett County, GA.|
HOT lanes take advantage of unused capacity in HOV lanes by allowing vehicles that do not meet the minimum occupancy requirement to pay a toll to use them. The facility operator can set a regular toll schedule, changed by time of day or day of the week, or change the price dynamically in response to congestion. HOT lanes use both vehicle eligibility and pricing to regulate demand.
An example of a successful managed lane facility is the I-15 Express Lanes project in San Diego, CA. The project utilizes an electronic statewide toll collection system called FasTrak, which enables drivers to prepay their tolls, eliminating the need to stop at a toll plaza. Prior to the addition of the Express Lanes, which are still under construction and anticipated to be completed in 2012, the heavily congested I-15 corridor included two reversible express lanes in the median. Concrete barriers separated these lanes from the general-use lanes. The express lanes, when first opened, were restricted to HOVs carrying two or more people, which left the lanes underutilized while the adjacent mainline lanes were heavily congested. In the late 1990s, single occupant vehicles were allowed to use the reversible lanes for a fee in an attempt to reduce congestion on the mainlines. Now, with the expansion of the system, the new lanes are managed with the use of movable median barriers to optimize the Express Lanes under varying traffic conditions throughout the day.
Key geometric design-related topics for managed lanes that require further research include lane separation, access, and communication with drivers. Human factors research also is needed to investigate the decisionmaking process for drivers entering a managed lane facility who are unfamiliar with these designs. Elements such as signing and providing roadway geometry that enables appropriate decisionmaking are critical, especially when using multiple operating strategies that can change the eligibility of vehicles using the managed lanes over the course of a day or week.
Design of entry and exit points is a critical issue that affects the operating conditions of a managed lane facility as well as the ability to modify operating strategies. Current HOT lanes generally have distinct separation and limited access points. Even with physical separation, greater frequency of access to managed lanes can degrade the ability to manage acceptable service levels. The spacing of at-grade ramp connections and length of weaving sections could affect safety and operating conditions of both the managed lanes and adjacent freeway lanes. Terminal access to the managed lanes also can be a design challenge, particularly in fitting the facility within an existing freeway or arterial street system.
For managed lanes that use pricing, the need for manual and automated enforcement is a critical design consideration. Separation of the lanes from adjacent general-purpose lanes is another challenge. Agencies can separate lanes in several ways, such as with painted stripes, plastic pylons, or concrete barriers. Lane separation is a safety concern because the parallel traffic could operate at very different speeds during congested periods.
How to clearly communicate about managed lanes to drivers while avoiding information overload requires careful evaluation as well. Vital information can include entry and exit points, occupancy requirements, operating hours, and toll amounts. Drivers must process this information, along with standard directional and informational signage, and still operate their vehicles safely. Drivers also might need to decide whether to use the facility, which can be particularly challenging for those unfamiliar with the corridor.
In addition, agencies considering managed lanes need to account for potential changes to user groups or varying tolls based on user groups. Management strategies such as charging based on distance or access point, or a combination of both, will present further challenges when designing a facility that can accommodate various operational strategies. Designing for complex operational scenarios also will require consideration of multiple tolling and enforcement zones.
Design Policy Changes
Various organizations are readying guides for publication in 2010 on policy and tools for making geometric design decisions. Some of the guides will reflect the continuous evolution of design practice, while others will be a significant shift in approach.
A Policy on Geometric Design of Highways and Streets. The next edition of this AASHTO publication, commonly referred to as the Green Book, will include information on design flexibility, rumble strips, and roundabouts. In terms of flexibility, the guide will highlight opportunities for highway designers to consider context when selecting design criteria. Applying a limited set of design values tends to favor one type of user over another -- cars, trucks, transit, pedestrians, or bicyclists -- and is not appropriate for every setting or type of road.
The new Green Book also looks at 2+1 roadways, which use an additional center lane for passing and provide alternating opportunities for each direction of travel. These roadways generally operate more safely and efficiently than conventional two-lane highways serving the same traffic volumes.
Rumble strips are another expanded topic. These raised or grooved patterns in a pavement cause a sound and vibration discernible to motorists driving over them. When constructed along the edge or shoulder of a roadway, rumble strips reduce run-off-the-road crashes. When installed along the centerlines of rural highways, they help reduce head-on collisions. Rumble strips placed transverse to the roadway alert drivers to use caution when approaching toll plazas, horizontal curves, intersections, and work zones.
The Green Book also addresses design considerations for roundabouts, which DOTs now are installing in greater numbers in the United States. The geometry of approaches and vehicle paths around the circulatory roadway is critical to roundabouts' smooth operations.
Roadside Design Guide. This AASHTO publication focuses on the "forgiving roadside philosophy" -- safety treatments that minimize the likelihood of serious injuries when motorists leave the roadway. The 2010 guide will include a new chapter on low-volume roadways, with guidance on clear zones, drainage placement, slope and ditch cross sections, barriers, sign supports, and utility-pole placement. A revamped chapter on roadside safety in urban or restrictive environments is based largely on NCHRP Report 612 Safe and Aesthetic Design of Urban Roadside Treatments.
Doing more with less requires making the most of technological innovation to improve geometric design practices. One of the newest tools for highway engineers is design visualization. Increasingly, engineers are synthesizing traditional two-dimensional (2-D) plans into various types of 3-D and dynamic (animated or real-time simulation) 4-D models, renderings, and simulations.
Novel Approaches to Intersections and Interchanges
With almost one-quarter of all U.S. fatal crashes occurring at intersections, FHWA and its partners are exploring alternative intersection and interchange designs to improve safety, capacity, and traffic flow.
Roundabouts. A modern roundabout is a one-way circular intersection, with no traffic signals or stop signs, where drivers travel counterclockwise around a center island. At entry, drivers yield to traffic already in the roundabout, then exit at their desired streets. By minimizing traffic conflicts, particularly left turns, a roundabout can be safer and more efficient. FHWA studies show roundabouts can increase traffic capacity by 30-50 percent compared to traditional intersections.
Displaced left-turn (DLT) intersections. Also known as a continuous-flow intersection, a DLT eliminates potential conflicts between left-turning vehicles and oncoming traffic by adding a left-turn bay to the left of oncoming traffic prior to the main intersection. Vehicles access the bay upstream of the main signalized intersection and cross over the median and opposing through segment. Agencies also can implement DLTs at diamond interchanges.
Double crossover diamond (DCD) interchange. Also referred to as a diverging diamond interchange, this design accommodates left turns at signalized, grade-separated interchanges of arterials and limited-access highways while eliminating the need for left-turn phasing. At a conventional diamond interchange, drivers execute left turns across the path of opposing through traffic. Flipping the traffic streams within the interchange area removes the conflict between the left turn from the major road and the opposing through movement. Research shows that DCDs increase capacity by up to 30 percent, reduce construction and right-of-way costs by 30-50 percent, and improve safety.
Quadrant roadway (QR) intersection. This intersection includes an extra roadway between two legs of the intersection that provides a separate connection between the major road and crossroad. Drivers wanting to turn left from the major road at a conventional intersection would actually turn left onto this connector roadway upstream from the major intersection, then turn left again from the connector roadway to the cross street. The QR in its purest form removes all left turns from the primary intersection to maximize through traffic on both the major and minor intersecting roadways.
Reverse jug handle left turns. One of the various forms of the New Jersey jug handle, the reverse jug handle is essentially a small loop at the far side of an intersection, where left-turning traffic can instead pass through the intersection and make two rights.
Median U-turn intersections. Also called the Michigan left turn, this design is similar to the reverse jug handle in that left turns are not allowed at major intersections. Left-turning vehicles instead pass through the intersection, make a U-turn downstream, travel back toward the intersection, and turn right onto the crossroad. This type of treatment is most effective on streets with wide medians.
Angled positive offset left turns. Slightly modifying the design of conventional left-turn lanes achieves a positive offset between opposing left-turning vehicles. This modification facilitates enhanced sight distance for vehicles, enabling drivers to better judge approaching vehicle gaps.
Restricted crossing U-turn intersection. Also known as a superstreet intersection, this design is similar to the median U-turn left turn, but through and left-turn maneuvers are not allowed from the side road. Instead, traffic must turn right on the major road then travel to make a U-turn. This design is most applicable for arterial segments.
For more information about these alternative intersection and interchange treatments, visit www.fhwa.dot.gov/safety.htm.
Until recently, designers used visualization primarily as a conceptual exhibit at public meetings. But advances in personal computing and development of computer-aided design and drafting have helped put design visualization directly into the hands of highway designers. In fact, visualization can improve the entire planning, design, and construction process for all types of projects, big and small, from start to finish.
Design visualization offers numerous benefits. In addition to enhancing public involvement, it enables engineers to examine their own concepts from multiple viewpoints. The interaction of design elements becomes more apparent. Engineers can identify and communicate anomalies and conflicts embedded in designs, such as inconsistent slopes, relationships with structures, drainage problems, and utility conflicts. Visualization also can be an effective method for quality control and quality assurance.
Visualization enables designers to analyze safety issues from multiple perspectives including that of the end user, whether a driver, bicyclist, or pedestrian. Among the advantages is enhanced analysis of sight distances for stopping, passing, intersection navigation, and directional decisionmaking.
Transportation agencies are beginning to use visualization as a tool for RSAs during preconstruction planning. RSA teams typically visit project sites to view existing conditions firsthand but must rely on 2-D drawings to assess safety issues of proposed design improvements. Visualization helps teams better understand designs and users' perspectives.
Visualization has become an important tool to help designers ensure that the physical layout of a roadway is recognizable to all users and is intuitive to navigate. By applying 3-D modeling and visual analysis, engineers can reduce the complexity of interchanges and intersections and make them easier to recognize and navigate, leading to safer and more efficient operations.
In 2005, crashes were the leading cause of death for people ages 3 through 6 and 8 through 34. Current design practice does not fully achieve desired safety goals, indicating that improvements in design methodology and practices are needed. Safety objectives are becoming more quantifiable in the design process. New tools and methodologies are becoming available to evaluate the predicted safety performance of proposed designs, which will enable designers to consider safety more accurately and explicitly in design decisions.
"Doing more with less can mean achieving greater performance from our transportation system," says FHWA's Merritt. "We have to do this despite ever-growing numbers of motorized and nonmotorized users, greater movement of people and goods, and increasing vehicle-miles traveled."
Geometric design is fundamental to development and improvement of the Nation's highway system. Whether addressing safety problems, reducing congestion, or meeting the needs of a growing and diverse population and user base, the choices that designers make are essential for success. Geometric design innovations are giving transportation professionals the tools they need to do more with less.
Highway Safety Manual(HSM). Jointly developed by AASHTO and the Transportation Research Board (TRB), this 2010 manual contains new tools for quantitative and substantive estimation of safety impacts of design decisions. HSM advances analytical prediction tools to improve decisions on roadway planning, design, operations, and maintenance based on explicit consideration of their safety consequences.
Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT # 400). This 2001 AASHTO publication provides examples of performance- and risk-based approaches to design. The guide acknowledges the lower risk associated with very low-volume (400 vehicles per day or fewer) facilities and provides criteria to achieve optimum performance. Because most U.S. roads are very low-volume, two-lane roads that are unlikely to be high priorities for major investment, the guide is especially helpful for enabling DOTs to stretch scarce resources to correct safety problems.
Mitigation Strategies for Design Exceptions(FHWA-SA-07-011). This 2007 FHWA publication deals with performance- and risk-based approaches to design. The guide examines the safety and operational tradeoffs of design elements that do not fall within the acceptable range of values in design criteria and standards.
NCHRP Reports 600A and 600B, Human Factors Guidelines for Road Systems, Collection A,and Human Factors Guidelines for Road Systems, Collection B. Both documents describe human factors principles and findings for consideration by highway engineers. The documents help engineers consider roadway users' capabilities in the design and operation of highways.
|Design visualization, such as this computer simulation of the impacts of a movable median barrier on California's Golden Gate Bridge, enables designers to analyze safety issues from more perspectives than is possible using traditional 2-D drawings.|
Patrick Hasson is the manager of FHWA's Resource Center Safety and Design Technical Service Team. He has been with FHWA as a safety and highway design engineer and team leader since 1987. He has a B.S. in civil engineering from the University of Maryland and an M.S. in engineering from Cornell University.
Steve Moler is a public affairs specialist at FHWA's Resource Center in San Francisco. He has been with FHWA since 2001, assisting the agency's field offices and partners with media relations, public relations, and public involvement communications. He has a B.S. in journalism from the University of Colorado at Boulder.
Many of the practices, treatments and approaches discussed in this article were presented in the report Improving Safety, Mobility, and Livability with Better Geometric Design Practices at the 4th International Symposium on Highway Geometric Design, held June 2-5, 2010, in Valencia, Spain. More information about the symposium is available at www.4ishgd.valencia.upv.es.