Shouldering The Load
The use of paved shoulders as temporary travel lanes adds capacity when it's needed most.
Traffic congestion during peak periods is common on many urban freeways throughout the United States. The main cause? Increased traffic demand. According to the Federal Highway Administration (FHWA), between 1980 and 2003, annual vehicle-miles traveled increased by 89 percent, while total road miles grew by just 3 percent. The volume of traffic using many of the Nation's roadways now exceeds the capacity of the existing infrastructure.
As the discrepancy between vehicle-miles traveled and available capacity -- as measured in freeway lane miles -- grows, the problem of peak congestion is only worsening. In fact, researchers with the Texas A&M Transportation Institute, in their 2012 Urban Mobility Report, found that travel delay in 498 U.S. urban areas increased from 1.1 billion hours in 1982 to 5.5 billion hours in 2011.
A number of factors limit the Nation's ability to build its way out of this problem. The lack of right-of-way, scarcity of funding, and environmental concerns limit construction of higher capacity facilities. Recognizing that these factors are unlikely to change in the near future, many metropolitan areas have begun to focus their resources on improving the flow of traffic on existing infrastructure.
To this end, departments of transportation (DOTs) are rolling out traffic management programs. The core elements of improving the efficiency of traffic flow are traffic management centers and intelligent transportation systems (ITS). Studies by FHWA and others have shown these deployments to be effective in reducing congestion related to incidents and planned special events, but tackling the recurring congestion that results when demand exceeds capacity remains an ongoing challenge.
One potential solution, already widely used in Europe, is the use of paved left or right shoulder lanes as temporary or interim travel lanes. Also known as hard shoulder running, this strategy can offer critical additional capacity to reduce recurring congestion. Researchers at FHWA recently studied the use of hard shoulder running overseas and in several applications here in the United States. What follows is a snapshot of how hard shoulder running works, sample U.S. deployments, and key operational and safety considerations.
A Tool for Active Traffic Management
The next generation of traffic management philosophy has begun to emerge under the banner of active transportation and demand management. The principles underlying this philosophy are to implement strategies to improve the efficiency of traffic flow in conjunction with strategies that influence or adjust demand. The two sets of measures thus work in concert to help balance supply and demand.
Among the most prevalent strategies used to improve traffic flow is active traffic management. Deployments in Europe typically involve the use of overhead lane control signals to manage traffic flow. Road managers use the signals to display variable speed limits that help smooth traffic flow through congested conditions in an orderly manner. They also can display messages regarding dynamic lane closures and merge warnings when conditions warrant. With the ability to manage speeds in a harmonized manner, road managers can minimize instances of traffic flow breakdown caused by stop-and-go conditions that typically occur once a freeway reaches the level of saturation. In addition, they can provide advance warning to motorists regarding backups due to major incidents.
As reported in FHWA's November 2010 Efficient Use of Highway Capacity Summary: Report to Congress (FHWA-HOP-10-023), Europeans have used hard shoulder running for years in conjunction with overhead lane-control signals and variable speed limits in lane management systems. The dynamic use of breakdown shoulders as travel lanes helps add capacity to the system during times of critical need.
"By strategically identifying locations, using lane assignment technologies, and monitoring performance needs, the addition of temporary capacity through the use of shoulders as travel lanes will allow the active management of our roadways in a safe and efficient manner," says Robert Arnold, director of FHWA's Office of Transportation Management. "Europe has used hard shoulder running successfully for years as part of a suite of strategies to better manage its roadways."
In some cases, European transportation agencies have even retrofitted freeways with additional breakdown or pullout areas. These areas provide a safe refuge for vehicles that experience difficulties or are involved in minor incidents during periods when the shoulders are carrying traffic.
To date, in the United States, the primary use of shoulders as temporary travel lanes has been by public transit buses that are bypassing slow traffic in the general-purpose lanes. Typically, this practice involves designating specific times of day that the shoulders operate as bus-only lanes.
The Minnesota Department of Transportation (MnDOT) began using bus-only lanes in 1992 and now has implemented the strategy on most freeways in the Minneapolisâ€“St. Paul region. To date, MnDOT has more than 310 miles (500 kilometers) of shoulder lanes for use by buses during designated times of the day.
According to MnDOT officials, use of these shoulder lanes has improved the ontime performance of the bus system significantly by providing reliable travel speeds at all times of the day and has made a significant contribution to increased transit usage across the metro area. The operating rules of the system allow buses to travel up to 15 miles per hour (mi/h) (24 kilometers per hour, km/h) faster than traffic in the adjacent general-purpose lanes, up to a maximum of 35 mi/h (56 km/h). If traffic is flowing at 35 mi/h (56 km/h) or faster, the buses simply stay in the general-purpose lanes. As this usage spread throughout the metropolitan area, MnDOT developed standards and guidance for use of the shoulders by the transit buses.
"Allowing buses to use the shoulders is a great example of agencies coming together, exploring options, and coming up with an inexpensive solution to getting the most people through congestion on existing infrastructure," says Carl Jensen, team transit manager at MnDOT. "Transit agencies that use bus shoulders have more reliable routes, which encourages more riders to use the bus. For MnDOT, more riders on the bus means [fewer] vehicles on the highways, which has many benefits including less congestion and pollution."
In addition to Minnesota, at least eight other States have implemented similar bus-on-shoulder applications: California, Delaware, Florida, Georgia, Illinois, Maryland, New Jersey, and Virginia. FHWA's Evaluation of Operational and Safety Characteristics of Shoulders Used for Part-time Travel Lanes (FHWA-HOP-12-008) notes the following examples of benefits for these types of applications:
- Iâ€“805/(SR 52) Connector in San Diego, CA: 5-minute travel-time savings and 99 percent ontime performance for buses using this 5.5-mile (8.9-kilometer) segment
- SR 400 in Georgia: 5- to 7-minute travel-time savings on this 12-mile (19.3-kilometer) segment
As noted in FHWA's Efficient Use of Highway Capacity Summary: Report to Congress, this operational strategy is generally a low-cost and quickly implemented solution that does not require costly expansion of highway right-of-way. Agencies can implement bus-on-shoulders programs on both highway and arterial corridors, but arterial applications often must rely on additional operational treatments, such as signal prioritization, in order to maintain a time advantage over regular traffic.
Fixed-Time Shoulder Use for All Traffic
To date, only a limited number of State or local agencies in the United States have opened the shoulder lanes to all traffic temporarily. Among those with the longest history are applications in Boston, MA, and northern Virginia. Each of these applications involves a peak-hour usage of the shoulder lane and use of roadside signage with specific times shown to alert motorists as to when the shoulder lanes are open to traffic.
Three freeways in Boston, Iâ€“93 (two segments), Iâ€“95, and (SR 3), allow the use of the shoulders during specific time-of-day operations. The inbound direction toward central Boston operates from 6 to 10 a.m. weekdays, while the outbound directions operate from 3 to 7 p.m. Only heavy trucks are prohibited from using the shoulder lanes.
When the Massachusetts Department of Transportation (MassDOT) began using the treatment in 2002, congestion on these facilities was so severe that traffic was at a standstill during peak periods, and frustrated drivers al-ready had begun using the shoulders illegally. Today, MassDOT, the Massachusetts State Police, and the Commerce Insurance Courtesy Patrol assist with the operation of the shoulders.
With each deployment, MassDOT sought approval from FHWA to use the shoulder strategy as a temporary measure until the department could obtain funding and approval for widening the roadway to add a permanent travel lane. MassDOT constructed each shoulder lane to the standards of a traditional widening project, with drainage being moved to the new edge of the pavement, and guardrails and fixed-object shielding being shifted accordingly. The typical design is a 10-foot (3.1-meter) minimum width and a 12-foot (3.7-meter) desirable width. Crews removed the scored concrete, rumble strips, and block pavers in the process. To provide refuge in case of incidents, MassDOT installed emergency pullouts approximately every 0.5 mile (0.8 kilometer) along the deployment sections.
In northern Virginia, a temporary shoulder lane application along Iâ€“66 just outside Washington, DC, was first implemented in 1992 between U.S. 50 and Iâ€“495. It operates during specific hours as a shoulder use lane for all traffic. The eastbound lane operates from 5:30 to 11 a.m. weekdays, while the westbound lane operates from 2 to 8 p.m. Static signage indicates operating hours, while overhead signs above the shoulder lane display either a red "X" or a green downward arrow to inform motorists when the lane is closed or open to traffic, respectively.
Recently, two other notable shoulder use applications have begun operation as well: U.S. 2 in Seattle, WA, and SR 400 in Atlanta, GA. Both sites are open to all traffic and operate on a time-of-day fixed schedule. The SR 400 location initially opened as a bus-only shoulder lane in 2005 but was later opened to all traffic in 2012.
Dynamic Shoulder Use
At this time, only one application of dynamic shoulder use, somewhat similar to those in Europe, is in operation in the United States. On Iâ€“35W in Minneapolis, MnDOT operates a priced dynamic shoulder lane on a 1.6-mile (2.6-kilometer) section approaching downtown.
This application is distinctive in a couple of ways. First, when open to traffic, this lane is an extension of a price-managed lane. The recent expansion of Iâ€“35W extends the high-occupancy toll (HOT) lane in both directions for 18 miles (29 kilometers) south of downtown Minneapolis. During peak flows, the HOT lane permits carpool and transit vehicles to use it for free, while drivers of single-occupant vehicles can pay a toll to use the lane. In the northbound direction, right-of-way limitations precluded any widening on the last 1.6 miles (2.6 kilometers) into downtown. Thus, the shoulder is opened as a travel lane for this section.
"Using the inside shoulder as a continuation of the HOT lane greatly improves the efficiency of the lane because it now extends to a major multilane exit into the downtown arterial network rather than ending 1.6 miles [2.6 kilometers] short and becoming a bottleneck in the freeway network," says Brian Kary, freeway operations engineer with MnDOT. "In addition, we were able to build this project for $17 million versus the full reconstruction to add a lane, which was estimated at $400 million. Also, the lane was added without increasing the freeway footprint, taking any right-of-way, or building any new infrastructure."
The other notable aspect of this application of shoulder use is the fact that MnDOT integrated it with the active strategies of lane management and variable speed limits, in a similar way to those applications in Europe. This combination enables MnDOT to open and close the lane dynamically when conditions warrant.
The U.S. Department of Transportation funded this first-of-its-kind deployment in the United States as a demonstration project under an Urban Partnership Agreement with MnDOT and the Twin Cities Metropolitan Council. The grant included funding for a detailed evaluation of the project's effectiveness, scheduled to be released in late 2013.
The Iâ€“35W application also is the only example of the left shoulder lane being used for this purpose on a freeway in the United States. The left shoulder operation is better suited for longer distance trips because it avoids having drivers navigate through areas where weaving occurs at entrance and exit ramps, which normally occurs to the right of travel lanes. The most complex design issue with a left-side shoulder use lane is finding an optimal way to terminate the lane while maintaining proper lane balance. In Minneapolis, the lane conveniently transitioned to a general-purpose lane prior to a major fork into the downtown arterial network. The creation of a bottleneck would have offset any upstream capacity benefits.
Right-side shoulder use lanes tend to have operational characteristics that are similar to those of auxiliary lanes, such as turn or merge lanes. Some of the deployments in the United States have continued the shoulder lane through interchanges, while others have opted to make the shoulder lane an exit-only lane at the interchange. DOTs typically implement bus-only shoulder lanes on the right side to allow for easier merging on and off the freeway.
FHWA's Evaluation of Operational and Safety Characteristics of Shoulders Used for Part-Time Travel Lanes (FHWA-HOP-12-008) found the functional capacity of most shoulder lanes to be approximately one-half that of a normal general-purpose lane. The limited functional capacity may be due in part to the geometric deficiencies, such as narrow width, close proximity of fixed objects, or lack of continuity associated with the lane, and to the fact that motorists may feel uncomfortable using the shoulder as a temporary travel lane. Speeds in the shoulder lane also tend to be 5 to 10 mi/h (8 to 16 km/h) slower than the adjacent general-purpose lanes. Research is underway at FHWA that aims to provide modeling capabilities for shoulder lanes, thus providing a potential benefitâ€“cost analysis for future applications.
Until more specific guidance is available on how to design shoulders for use as temporary travel lanes, DOTs can look to the American Association of State Highway and Transportation Officials' (AASHTO) publication, A Policy on Geometric Design of Highways and Streets, commonly referred to as the Green Book. This publication provides general guidance on the geometric dimensions of roadways, including the widths of travel lanes, shoulders, and clear zones. Another AASHTO publication, A Policy on Design Standards -- Interstate System, offers guidance related to standards for designing interstate highways.
When using the shoulder as a travel lane, the design standards need to be revisited. There will likely be no shoul-der adjacent to the temporary travel lane. In addition, the vertical and horizontal offsets will need to be recalculated with the new edge of the travel lane being on what used to be the shoulder. A request for a design exception should consider the design, safety, and operational aspects of the proposed application.
Safety data on existing U.S. deployments are limited. The Virginia Department of Transportation (VDOT) has performed the most extensive safety analysis to date, focused on the Iâ€“66 application. VDOT reported on the analysis from this deployment in a 2007 article in the Transportation Research Record titled "Safety Impacts of Freeway Managed-Lane Strategy: Inside Lane for High-Occupancy Vehicle Use and Right Shoulder Lane as Travel Lane During Peak Periods." Also, VDOT's 2009 study "Seasonal and Operation Hour Extension Effect on Traffic Congestion: A Study of Northern Virginia's Interstate 66 Shoulder Travel Lane Practice" shared additional insights. The safety and operational results from these studies contributed to VDOT's decision to expand use of the Iâ€“66 shoulder travel lane to a dynamically triggered operation that will be implemented in response to congestion levels along the corridor. VDOT plans to deploy this tactic in conjunction with other advanced traffic management strategies, such as overhead lane control signals and variable speed limits.
In addition to reviewing crash rates, FHWA recommends that DOTs consider the potential effects on incident response. Many first responders use the shoulder as a way to bypass traffic queued at an incident. Some DOTs have mitigated this problem by providing additional motorist service patrols within the corridor.
The use of shoulders as temporary travel lanes is one of several strategies that researchers with the FHWA Office of Operations are studying under the formative program area of active transportation and demand management. The main focus areas for temporary shoulder usage are in conducting research in design, safety, operational characteristics, and modeling. In addition, the researchers are conducting a comparative analysis of European evaluations and research related to hard shoulder running. The resulting information will help FHWA shape any future policy or guidance documents.
Gregory M. Jones is a transportation operations specialist who splits his time working with the FHWA Resource Center in Atlanta, GA, as well as with the Office of Operations in Washington, DC. He has worked for FHWA since 1984. Jones provides national technical support in the areas of congestion pricing, managed lanes, active transportation and demand management, emergency transportation operations, intelligent transportation systems, freeway management, and regional operations partnerships. Jones has a B.S. in civil engineering from the University of Tennessee.