Efficient Use of Highway Capacity Summary
Report to Congress
Chapter 3: Critical Issues
The primary concern related to shoulder use involves its impact on operations and safety. The purpose of this project was to assess the deployments of shoulder use in the United States and determine the impacts on, the experience with, and the addressing of the following safety issues:
- Conflicts at access ramps.
- Sight distance.
- Conflicts with motorists pulling onto the shoulder for emergency purposes.
- Loss of use of the shoulder for emergency refuge.
- Need for bus driver training.
- Speed differential between the general purpose lanes and the shoulder.
- Effect on general purpose lane users.
- Return merge distance adequacy.
- Debris hazards on shoulder.
- Reduced bridge clearance.
- Operational efficiency.
- Crash experience.
By providing a summary of this information and data related to shoulder use experience, it is expected that the project can assist FHWA with developing guidance on the subject and identifying what, if any, research needs to be conducted to address unknowns related to the operational strategy and its potential long-term and widespread use in the United States.
Through the course of this effort, the project team conducted a thorough informational scan and literature search to identify the existing state of the practice and relevant traffic and safety information associated with safety shoulder usage. Furthermore, they conducted personal telephone interviews to seek additional direct input from various agencies that are known for their use of shoulders for interim or long-term congestion relief. The study regions interviewed were Minneapolis-St. Paul, Virginia, Massachusetts, and Washington. The following sections highlight the critical operations and safety issues related to shoulder use that arose from these activities. Individual case studies on several operational facilities are included in Chapter 4. The interview questions used by the project team are included in appendix A.
It is always desired to have a minimum 12-ft lane width for all freeway travel lanes. A reduction in this lane width reduces the overall capacity and may impact smooth operations. However, with regard to temporary shoulder use, narrower lane widths can be acceptable due to the limited use and operating conditions during their use. For example, the Mn/DOT uses a minimum 10-ft shoulder as an acceptable width on those facilities where BOS is operational. (18) Where the BOS travels across a bridge, 11.5 ft is a required minimum, with 12 ft being the minimum for new construction that may accommodate BOS. The narrow width does not adversely impact operations given the lower speeds of the transit vehicles (35 mph maximum) and controlled speed differential (buses can travel no more than 15 mph faster than the main lane traffic). Additionally, Mn/DOT restriped the shoulder on I-94 to 11 ft when converting the shoulder after the collapse of the I-35W bridge. (24) Additionally, the shoulder on I-35W where Mn/DOT has deployed priced dynamic shoulder lanes on a former BOS left shoulder ranges in width from 17 ft to 19 ft from the barrier to the edgeline stripe.
The temporary shoulder use on I-66 in Virginia does have 12-ft lanes throughout the facility. (25) No special concerns or accommodations have been needed for over-width vehicles in some States. (18), (24), (26) However, in Massachusetts, no heavy trucks are allowed on the shoulder, and only two-axle vehicles weighing less than 26,000 lbs can use the MnPASS PDSL on I-35W. In Virginia, there are some locations along I-66 where a small amount of pavement, ranging from 1 ft to 12 ft, remains outside of the shoulder to help with wider vehicles. While this width is not huge, it can be helpful in some circumstances. In Massachusetts, the minimum shoulder width required is 10 ft with a desired being 12 ft. (23) The WSDOT restriped the US 2 trestle to provide a 2-ft left shoulder, two 11-ft lanes, and a 14-ft right shoulder where the shoulder use is permitted. (26) With the I-94 installation in Minnesota, the shoulders were not widened prior to allowing all vehicles to use the shoulder. The agencies were confident that the shoulders could handle the additional traffic without compromising the pavement integrity. With the Mn/DOT BOS deployments, each installation is unique. Thus, if the agency determines the shoulder can handle the additional bus traffic, no pavement widening is undertaken. For other situations, Mn/DOT has developed cost estimates for BOS deployments if shoulder improvements are needed, recognizing that all facilities may not be able to handle the additional traffic.
One of the potential challenges with using shoulders for travel lanes is the presence of fixed objects within the recovery area of the shoulder. With the Mn/DOT BOS applications, fixed object shielding is not installed since the shoulders are only used when speeds are low and congestion is present. In some cases, though, the agency has had to move guardrails to provide the 11.5-ft minimum pavement width. If an object is such that it cannot be moved, Mn/DOT requests a design exception for the case. Along the I-94 shoulder use installation, some fixed object shielding was installed because of ramp reconfigurations, and some retaining walls were installed to accommodate the widening where the shoulder was not adequate in width. In Virginia on I-66, some barriers were extended to address fixed object issues. Furthermore, some deceleration lanes were shortened and acceleration lanes extended for the same reason. In Massachusetts, each of the shoulder use deployments was treated like a traditional widening project where guardrails and fixed object shielding were moved to appropriate locations.
Treatment at Interchanges
In the domestic applications of temporary shoulder use, the vehicles traveling in the shoulder lane traverse across the entrance and exit ramps as they would if they were in the main lanes. (18), (23), (24), (25) Of interest is the BOS operating rules in Minnesota. On those facilities with BOS, buses must yield to any vehicle entering, merging within, or exiting through the shoulder. (18) Also, if the ramp meter at an entrance ramp is creating significant queues, many transit drivers will merge into the general purpose lane to allow the ramp traffic to merge without conflict with the buses. This maneuver is not mandatory of drivers, but those with significant experience normally execute it to enhance freeway operations. Once they pass the ramp, they move over onto the shoulder and proceed. In Germany and The Netherlands, vehicles on the shoulder continue on the shoulder through an entrance or exit ramp. However, in England, drivers on the shoulder must exit the motorway at the interchange, travel through the intersection at the junction, and reenter the motorway to continue traveling on the shoulder.
In general, agencies have made improvements or changes to drainage inlets on those facilities with shoulder operations. In Minnesota, those catch basins on the facilities with BOS are reinforced as a precautionary measure. They are also raised to be flush with the pavement so that operations are not impeded. (18) Along I-94, no additional drainage inlets were installed prior to opening the shoulder to all users, as the conversion was a weekend operation that needed a quick turnaround. With the I-66 installation in Virginia, some design exceptions, which were most likely approved as cross-slopes along the facility, are different than what is typical for other like roadways. (25) Other drainage issues that have arisen as a result of cross-slope issues have been resolved with the installation of additional inlets. In Massachusetts, projects were treated as traditional widening projects, and drainage inlets were moved to the new edge of pavement.
Drainage capacity needs to be reviewed for a design year storm. While some stormwater storage might be acceptable on a safety shoulder, the same is not the case on a travel lane.
Emergency Refuge Areas
Perhaps the most critical concern regarding the use of shoulders as travel lanes is the inability for vehicles to use the shoulder in the event of an emergency. This condition can be minimized by installing emergency refuge areas (ERAs) periodically along the facility. Along the M42 in England, emergency refuge areas are located approximately every 500 m and include emergency roadside telephones. The telephones are accessible to wheelchair users, located behind safety fencing, and feature text messaging and eight different languages. (30) With regard to the design of these ERAs, the entrance taper is 25 m and the exit taper is 45 m. In Virginia on I-66, emergency pull-outs are located wherever space was available. The lowest spacing is less than 0.5 mi, with the greatest spacing being 2.5 mi between pull-out areas. Additionally, there are a number of locations designated as "informal" pull-outs where traffic can pull out of the shoulder lane into a safe area. These areas are located where space permits and are often narrower than a formal shoulder pull-off. However, they do provide some refuge area that is better than none at all. Entrance and exit tapers are typically 300 ft along the corridor. In Massachusetts, emergency pull-out areas are located approximately every ½ mi, and those installed along I-35W in Minneapolis are located off the right shoulder to handle emergency stops while the left shoulder is under use. (24)
The Mn/DOT is one agency that has dealt with rumble strips along the facilities with BOS. On those roadways with rumble strips on the shoulder, designers either move the rumble strips so that the buses straddle the strips, or install rumble stripes on the pavement marking. In both cases, the transit vehicles avoid driving on the affected pavement. (18) The MassDOT removed rumble strips where the shoulder operations were deployed and also removed scored concrete and block pavers that would affect driving performance on the shoulders. (23)
The safety benefits from the U.S. applications have mostly been inconclusive. However, there are specific safety benefits noted in the European case studies. In fact, it is the safety benefits that have been the leading success stories of these applications. It should be noted that these shoulder use applications also have a major investment in active traffic management strategies. In addition, these systems require significant investment in ongoing maintenance, operations, and system control manpower.
Future U.S. studies should also address the freeway sections upstream of the shoulder use section to get the full appreciation of safety benefits. Many congestion related crashes happen well in advance of the actual bottleneck location as traffic queues as a result of the congestion.
The ITS components of the shoulder use deployments varied by location. (18), (23), (24), (25), (26) In Minnesota, components included those typical across the region, including cameras and loop detection. Virginia installed loops in the shoulder pavement and ensured that camera coverage was adequate in the corridor. The WSDOT did not install any additional components along the segment. Massachusetts has some sensors on I-93, along with cameras.
Traffic Control Devices
Most of the traffic control devices deployed with shoulder use are
regulatory in nature. The VDOT uses an overhead regulatory sign—displayed
in Figure 16—to indicate shoulder operations. The regulatory sign
is combined with a lane control signal to indicate when drivers may
use the shoulder. Other ground-mounted signs used on the installation
include a regulatory sign with hours of operation (Figure 17), a regulatory
sign noting the presence of an emergency pull-off (Figure 18), and regulatory
signs indicating the beginning and ending of the shoulder operations
Figure 16. Photo. I-66 Regulatory Sign with Lane
Control Signals (Photo Courtesy of VDOT).
Figure 17. Photo. I-66 Hours of Use Regulatory
Sign (Photo Courtesy of VDOT).
Figure 18. Photo. VDOT I-66 Regulatory Signs (Photo Courtesy of VDOT).
The Mn/DOT uses ground-mounted signs to indicate BOS operations. The
two signs most frequently used are the regulatory sign stating that
only buses are authorized to use the shoulder (Figure 19) and a warning
sign for transit vehicles indicating they should merge with traffic
(Figure 20). Pavement markings for all operations are no different than
normal shoulder edgeline markings. It is important to note that it is
not advisable for an agency to install the special use lane diamond
on the shoulder to indicate temporary shoulder use. (18)
This marking is commonly associated with HOV operations, so drivers
may assume that HOVs are allowed to use the shoulder at all times. This
assumption is of particular concern in Minnesota where only buses are
allowed to use the shoulders and only during peak periods of travel.
Figure 19. Photo. Mn/DOT BOS Regulatory Sign (Photo
Courtesy of Mn/DOT).
Figure 20. Graphic. Mn/DOT BOS Warning Sign (Image Courtesy of Mn/DOT).
The MassDOT uses a combination of regulatory and warning signs to indicate
that the shoulder is open to all users during specified times. As noted
in Figure 21, regulatory signs are used to indicate the hours of operation
and shoulder use prohibition during non-operating hours. The sign in
Figure 22 is displayed to warn drivers to expect vehicles in the right
breakdown lane during the shoulder use operating hours. Regulatory signs
are also used to indicate the location of emergency pull-off areas.
Figure 21. Photo. Regulatory Shoulder Use Signs
(Photo Courtesy of MassDOT). (23)
Figure 22. Photo. Warning Shoulder Use Sign (Photo Courtesy of MassDOT). (23)
The Mn/DOT uses overhead sign gantries with a combination of static guide and regulatory signs, lane control signals, and DMS panels to indicate the operational status of the PDSLs and the MnPASS fee for using the lanes. These signs are illustrated in Figure 12, Figure 13, and Figure 14.
The WSDOT uses a regulatory sign, as shown in Figure 23, to provide
information to the user indicating when the shoulder is open and when
it is closed. (26) While none of the signs used by the various
States are identical in nature, they all fall within the guidance provided
by the Manual on Uniform Traffic Control Devices (MUTCD). (31)
Moreover, the agencies are confident that motorist comprehension of
these signs is high when combined with public outreach.
Figure 23. Drawing. Shoulder Use Regulatory Sign (Drawing Courtesy of WSDOT). (26)
There has been inconsistency in both the U.S. and Europe on the use of an outside edge line on the shoulders used as travel lanes. This is an area for further research and analysis.
Performance evaluation of shoulder lanes principally derives from two domestic sources—National Cooperative Highway Research Program (NCHRP) Report 369 (32) and an independent evaluation of I-66 (33) —and one international source—the M42 Program Evaluation. (34) The findings from these sources are summarized here along with other information provided by Mn/DOT, MassDOT, and WSDOT related to their facilities.
The NCHRP 369 provided a comparative benchmark analysis methodology of eleven corridors in six States, using in-corridor comparisons of "unaltered" segments (full shoulders with 12-ft lanes) and "altered" segments (use of shoulders with or without narrow lanes). (32) The evaluation hypothesis postulated that the lack of shoulders and/or use of narrow lanes would result in different operating conditions.
When isolated by level of service (LOS) categorizations, travel speeds on segments with use of shoulders/narrow lanes were not significantly different from their interstate standards brethren in low-volume and high-volume applications. However, there was a minor difference in speeds in medium-volume applications:
- Level of Service A/B: With volumes less than 1,600 vehicles per hour per lane (vphpl), speeds were identical between altered and unaltered facilities.
- Level of Service C/D: A slight decrease (less than 5 mph) in average travel speeds was found in altered facilities when volumes ranged between 1,600 vphpl and 2,000 vphpl. This was the only statistically significant difference.
- Level of Service E/F: Like LOS A/B, there was no difference in speeds between altered and unaltered facilities at volumes higher than 2,000 vphpl.
At LOS C or worse, there was no significant difference in the choice of lanes (shoulder vs. static lanes) by travelers. However, at LOS A or B, the use of shoulder lanes was significantly less prevalent, especially if the surface of the shoulder was different from the travel lanes. On part-time shoulder facilities (such as SR 3 and I-93 in Massachusetts and I-66 in Virginia), if traffic remains slow and the time is outside the operational shoulder-lane time period, drivers may ignore the shoulder-use restriction and use the lanes anyway. This complicates the ability of the operating agency to recapture the shoulder as a refuge area. In addition to the difference in average speeds, altered facilities exhibit a greater range of speeds in LOS C/D conditions (30–70 mph) than unaltered LOS C/D conditions (50–70 mph).
Lateral clearance effects of altered facilities did have an impact on drivers, causing them to shy away from the barrier. The percent of traffic within a foot of the interior lane line (regardless of lane) was lower at altered sites than unaltered ones. As would be expected, inadvertent lane-line crossings per hour increased significantly with altered sites compared with unaltered ones.
When Mn/DOT converted the BOS on I-94 to all vehicles in response to the collapse of the I-35W bridge, traffic congestion improved along the facility. In both directions, the length of queues and duration of congestion improved to near pre-collapse conditions, and travel times improved as well. (18) One negative impact of the elimination of the BOS was on transit performance on the facility. While previously transit vehicles enjoyed reliable travel times on the shoulders, once other vehicles were allowed to use the shoulders, that advantage disappeared. Metro Transit lost revenues of approximately 1.3 million dollars per year in fixed guideway funds from FTA as a result of the dedicated lane being eliminated. In terms of BOS on other facilities in the Minneapolis region, transit vehicles maintain a reliable travel advantage.
While only recently deployed, the shoulder operations on US 2 in the Seattle area have greatly improved conditions for the regional commute along that corridor. The primary performance measure WSDOT is using is travel time, and delay has fallen from 8-10 minutes to 1-2 minutes on the facility. (26) Travel times are much more reliable on the facility, and throughput has increased on a previously congested ramp.
As early as the late 1970s, agencies experimented with using freeway shoulders to increase capacity. A 1978 study of restriping the US 59 main lanes in Houston, which encroached on the shoulder and increased capacity, yielded positive results in terms of operations and safety. (21) With the Houston implementation, operations were improved during peak travel periods, travel time and quality of operation improved, and accident frequencies and rates dropped in the year after the deployment. However, a similar study of inside shoulder removals in California did not yield any significant reduction in overall accident rates as a result of the shoulder removal. (20) Instead, the research indicated that the drop in accident rates was a result of a decrease in congestion due to the capacity increase.
Initial safety reviews in the 1980s of shoulder lane usage indicated that projects implemented for short distances to address specific problems often yielded a decline in accident rates. However, the NCHRP Report 369 introduced an alternate methodology to examine accident severity, time of day, type of accident, and characteristics to validate the initial findings. Corridors examined were I-395 (Virginia), I-5 (Washington), I-5 (California), I-85 (Georgia), and I-10 (California). (32) Statistical analysis indicates that in aggregate across the study corridors, there was no significant difference between altered and unaltered segments. However, significant increases in accidents (up to 36 percent more in some segments on I-5) occurred in one specific alteration: a combination of use-of-shoulders and narrow lanes for greater than 1 mi in length. Under these conditions, accident frequency increased significantly, as did sideswipe, nighttime, and truck accidents. (35)
On I-66 in Virginia, investigators found no significant impact of the combined managed-lane (HOV) and shoulder-lane (general purpose) operations on traffic crash frequency. The majority of crashes were rear-end collisions, which are typically a result of congested conditions. Authors hypothesized that advanced incident identification and clearance as well as enhanced dynamic messaging signs contributed to the lack of evidence that the system increased crashes. Since the implementation of BOS in Minnesota in 1993, only 21 collisions have occurred related to the operations, most of which were sideswipes or mirror hits on the bus.
While it is too early to confidently assess safety improvements along US 2 in Washington, anecdotal evidence shows that collisions at the conflict point just upstream of the shoulder usage have been reduced. (26) However, it is difficult to measure the true isolated impact of the shoulder use since its deployment coincided with the deployment of ramp metering on I-5, which feeds into the US 2 trestle.
A critical concern with the deployment of shoulder use is the reduction of the clear zone distance. While an agency may be able to move some fixed objects, guardrails, and other barrier treatments, such accommodation is not always possible. Thus, when traffic is allowed to travel in the shoulder, the effective clear zone is reduced to distances below the minimum allowed. As a result, agencies frequently have to seek design exceptions from FHWA. The potential long-term implications of these exceptions are not known and bear consideration for any possible deployments.
The implementation of shoulder lanes has yielded higher costs and more difficult maintenance activities. Maintenance-related issues as identified by the NCHRP Report 369 include:
- Under altered conditions, highway appurtenances such as signage, barriers, drains, and lights were closer to traffic and were damaged more often and more severely than under unaltered conditions.
- In order to conduct regular maintenance, additional personnel and equipment are needed to close lanes and provide adequate work area protection.
- Most incidents, from minor to major, require some action by personnel that involves shoulders, which in turn requires shoulders to remain closed until the incident is cleared, items are removed, or other action is completed. Estimates by personnel indicate that clearance time for incidents doubles with shoulder lane use.
- As emergency vehicles use shoulders to access scenes of accidents, delays in arriving on the scene have consequent increases in periods of congestion, secondary accidents, and clearance time.
Enforcement is not a problem on those Mn/DOT facilities with BOS. Most of the enforcement issues addressed by Minnesota State Patrol have more to do with ensuring that cars do not block the buses on the shoulder on purpose. (18) Visual inspection and using radar to clock the speeds of buses are the methods of enforcement. Those buses that are traveling faster than 35 mph while on the shoulder are reported to the transit agencies, and the drivers receive a reprimand. Never have repeat complaints about speeding been received by Metro Transit or Mn/DOT because the drivers realize that their job depends on following the operational rules and requirements.
Enforcement along I-66 is difficult for both Virginia State Police and county law enforcement personnel. The short distance and tight interchange spacing makes enforcement a challenge for officers wishing to cite a driver using the shoulder during the off-peak period. (25) However, the high off-peak violations do not present a safety problem because of the short distances between interchanges.
Enforcement activities in Massachusetts and Washington are typically handled by State police forces. The enforcement approach is visual inspection by the officers. These agencies report little problems with violators along the corridors where shoulder use is provided. (23), (26)
Incident response along the facilities with temporary shoulder use is handled in a similar manner for all of the agencies. A typical unified response approach is deployed; this response is led by State police and the DOT first responders and uses typical regional response protocols. (18), (23), (24), (25), (26) Average response times are typical for the shoulder use facilities as with other facilities in the regions. In Virginia, the safety service patrol was increased in the corridor, particularly during the operating hours of the shoulder on I-66. (25) Additionally, emergency pull-out areas were installed along I-35W in Minneapolis to help facilitate incident response during the time period when the PDSLs are operational. (27)
Metro Transit, the largest transit operator in the Minneapolis-St. Paul region, uses a training manual along with class time, route, and safety pamphlets for training its bus operators. The manual includes a small section on bus shoulder use, along with a training video and on-board training related to the BOS operations for the drivers. Drivers are trained on the Minnesota statutes (http://www.dot.state.mn.us/metro/teamtransit/docs/mn_statutes_2006.pdf), the Mn/DOT Commissioner Order (http://www.dot.state.mn.us/metro/teamtransit/docs/draft_order_7-29-05.pdf), the Mn/DOT Guidelines on Shoulder Use by Buses (http://www.dot.state.mn.us/metro/teamtransit/docs/bus_only_shoulder_guidelines.pdf), and the Mn/DOT Shoulder Operating Rules (http://www.dot.state.mn.us/metro/teamtransit/docs/operating_rules_on_shoulder.pdf). A copy of the Metro Transit Training Video is available at http://www.dot.state.mn.us/metro/teamtransit/visual/Training%20For%20Bus%20Drivers%202.wmv).
The installation of a temporary shoulder can be a reasonable expense when compared to the construction of new freeway lanes. While the initial costs to convert the shoulders on I-66 in Virginia are not known because of the lack of current institutional knowledge, a recent assessment to upgrade the lane control system along with the facility was estimated to be 7 million dollars for the entire 6-mi segment. While that installation is somewhat expensive, it is still much less than constructing 6 mi of new pavement in a congested urban area. Project costs are unknown for the Massachusetts projects. In Minnesota, the PDSL project cost 13 million dollars, which included a new pavement surface for the entire facility, including the shoulder lanes and emergency pull-out areas. Contrast those costs to the approximately 70,000 dollars it cost WSDOT to install a 1.5-mi shoulder segment on the US 2 trestle. The cost for Mn/DOT to install BOS along a freeway varies according to the requirements, but most installations are very reasonable, as shown in Table 1.
In the applications in the United States, no liability issues were
of noted concern at the time of installation and deployment. One concern
was broached by the American Automobile Association (AAA) with respect
to the extension of operating hours along I-66 in Virginia. (25)
When VDOT announced plans to extend operating hours to improve operations
with longer peak periods, AAA expressed concern because the hours in
which the shoulder would be available for emergency refuge in the event
of an incident would be reduced even further. However, with concerted
effort on the part of VDOT to work with AAA to address their concerns,
no issues of significance have emerged since the extension was implemented.
Table 1. Shoulder Use Installation Costs—Mn/DOT. (18)
||Costs Plus Signing & Striping
|Shoulder width and bituminous depth are adequate. Catch basins do not need adjustment. Signing and striping are only requirements.
||$1,500 per mile—Freeway
$2,500 per mile—Expressway
|Shoulder width and bituminous depth are adequate. Minor shoulder repairs and catch basin adjustments are needed.
||$5,000 per mile—Freeway
$5,000 per mile—Expressway
|Shoulder width is adequate but bituminous depth requires a 2-inch overlay. This assumes shoulder and roadway can be overlaid at the same time.
||$12,000 per mile—Freeway
$12,000 per mile—Expressway
|Same as above but adjacent roadway is not being overlaid. Shoulder must be removed, granular base adjusted, and increased bituminous depth replaced.
||$80,000–$100,000 per mile
|Shoulder width and depth replacement are required.
||$42,000–$66,000 per mile for both freeway and expressway
|Installing a 12-ft shoulder rather than a 10-ft shoulder in a new construction project.
||$30,000 per mile for both freeway and expressway
No specific legislation was required to implement the I-66 temporary shoulder use in Virginia. The implementation was under the jurisdiction of the Commonwealth Transportation Board and the Code of Virginia. (25) The Mn/DOT deployed the BOS under the order of the Mn/DOT transportation commissioner. Additionally, specific Minnesota statute (169.306) was changed to address the use of shoulders by buses and driving rules for those buses when operating on the shoulder (169.18). (18)
Public Outreach and Education
Initially, Mn/DOT did not implement a public outreach and education plan when deploying the BOS. Once the facilities were in operation, they did occasionally host a media event where a local celebrity challenged a transit vehicle to a race on a congested corridor with BOS during the peak period. The intent was to demonstrate the travel time savings and benefit to using transit in the corridor. These events received good media coverage and served as an educational opportunity for the general public. (18) Typical outreach efforts were used in the Seattle area prior to the opening of the US 2 shoulder lane. (26) The city of Everett, Washington, was a great proponent of the project because of the expected improvements for the commute out of the city.
NCHRP/AASHTO/FHWA Domestic Scan
In November 2009, an NCHRP/American Association of State Highway and Transportation Officials (AASHTO)/FHWA domestic scan was completed to look at the issue of maximizing flow on existing highway facilities. This scan tour visited and documented several temporary shoulder use locations around the United States. There were members of this team from FHWA and the following State DOTs: Washington, Georgia, New Hampshire, Michigan, and New Mexico. The results of the scan team's report are useful for development of guidance in temporary shoulder usage.
Agencies need to consider a wide range of issues when determining whether shoulder use is appropriate for a particular corridor or region. Experience both overseas and domestically provide a wealth of experience from which agencies can learn to make informed decisions. From the European perspective, part-time shoulder use is only used during congested periods when queues begin to build at bottlenecks in the system. Moreover, this treatment is almost always deployed in conjunction with speed harmonization. The intent is to reduce the speeds along the corridor and smooth out driver performance and reduce the likelihood of collisions. (1) Furthermore, European agencies have realized both safety and mobility benefits as a result of these projects. While American deployments have been limited, the experience has generally been positive. However, safety benefits have not been conclusive. Those issues that need to be considered include design, traffic control devices, performance measures, potential safety benefits, maintenance concerns, enforcement roles and processes, incident response, training for personnel, costs, liability and legal issues, and public outreach and education. Careful consideration of these issues can help ensure a shoulder use deployment is effective without having negative impacts on safety and operations.
Another area needing further analysis is the topic of left shoulder use versus right shoulder use. Domestically, almost all applications of part-time shoulder use have occurred on the right side, while shoulder conversions to permanent lanes have tended to be more prominent on the left side. Each application has a different subset of design and operational considerations to analyze.