Design Guide for Addressing Nonrecurrent Congestion (2014)

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89 4 CATALOG OF SECONDARY TREATMENTS This chapter presents a catalog of secondary treatments that can be considered for use in addressing nonrecurrent congestion. A detailed summary of each of the secondary treatments is provided that includes the following information: • Description and objective • Typical applications • Design criteria • How treatment reduces nonrecurrent congestion • Factors influencing treatment effectiveness • Cost The secondary treatments are classified based on similarities with respect to func- tion or location on the roadway. Chapter 3 presents a catalog of the nonrecurrent congestion design treatments. LANE TYPES AND USES Contraflow Lanes for Emergency Evacuations Description and Objective A contraflow lane is a lane that has been “borrowed” from one direction of travel to add capacity to the other direction of travel. This borrowing can be accomplished using overhead lane designations, static and dynamic signing, police traffic control, interchange gates or barriers, or by other means. Contraflow lanes can be used to ac- commodate emergency evacuations, most commonly in anticipation of hurricanes (as illustrated in Figure 4.1), although sometimes in cases of other natural or humanmade

90 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 4.1. Contraflow operations used in South Carolina after a hurricane. Source: Goodwin (2003). disasters. During evacuations, nearly all motorists using the facility want to travel in one direction—away from the threat of danger. In such situations, nearly all oppos- ing (i.e., minor-flow) lanes are converted to contraflow lanes, which essentially doubles the number of lanes available for evacua- tion traffic. Sometimes, one of the inbound lanes remains open for inbound travel by emergency or military vehicles. Typical Applications Contraflow lanes are most often used in situations when either an increase in demand or decrease in capacity necessitates finding additional capacity to serve vehicles in that direction. Common applications are during peak hours when demand is highly directional, during emergency evacuations, and during work zones. The first applica- tion is used in circumstances of recurrent congestion due to daily demand fluctuations and is outside the scope of this research. Contraflow lanes as a nonrecurrent congestion treatment during work zones are discussed in the next section. Contraflow lanes in emergency evacuations are consid- ered here. Applications of emergency contraflow lanes are typically found near coastal regions that experience hurricane (and therefore evacuation) risk. In addition, emer- gency contraflow may be used to evacuate residents from the path of a wildfire or flood or away from a terrorist attack. Design Criteria Many states within the United States have developed emergency evacuation plans for a variety of disasters, such as hurricanes. Several of these evacuation plans include contraflow lane systems to increase outbound roadway capacity. Emergency contraflow design plans include designating the contraflow routes and developing and installing appropriate emergency traffic control plans that include sign- ing, signal control, and expected levels of police (or other emergency worker) traffic control. In addition, ramps may be retrofitted with gates and devices to implement reversed traffic flow so that interchanges can effectively operate “backwards.” The most critical part of the plan is designing how the contraflow will terminate. Safety concerns in emergency evacuations are likely to be found at the terminal points (i.e., where the contraflow begins and ends) and at interchange ramps (where traffic in the wrong direction intersects with the arterial roadway network). Generally, a significant number of police officers or other officials are needed to manually direct traffic during lane reversals, especially at interchanges. It is important to remember that driver behavior will be different in emergency evacuations than under normal travel conditions, and therefore, the effective capacity of highway lanes may be reduced. In addition, the highway cross section may be

91 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION used differently in an attempt to increase capacity, such as by allowing travel on the shoulder or ignoring lane lines to facilitate an additional, narrower lane on the road- way segment. Finally, consideration should be given to the likelihood of additional traffic demand from nearby jurisdictions that are evacuating through the area being considered. How Treatment Reduces Nonrecurrent Congestion Increases Roadway Capacity During an Incident Using contraflow lanes during an emergency event increases the roadway capacity in the primary direction of travel. It is expected that in emergency evacuations, demand on outbound freeway segments will more than double and that even with full contra- flow plans in effect, evacuation routes will be operating with significant congestion. However, the increase in roadway capacity will reduce the overall evacuation time, ensuring that more people can evacuate areas of risk before harm comes. Although contraflow lanes will reduce congestion during an emergency evacua- tion, this reduction in congestion cannot be easily translated into an improvement in reliability. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of contraflow lanes at reducing non- recurrent congestion due to evacuations include the following: • Evacuation frequency • Treatment deployment frequency • Capacity available for contraflow during evacuation (may include shoulder use, if full-width, full-depth shoulders are available, and may exclude lanes reserved for emergency vehicles) • Expected demand during evacuation on the segment of interest (on the basis of availability of alternative routes and the desirability of the destinations served by the segment) Contraflow lanes for emergency evacuations will be most effective when demand volumes are high relative to the available roadway capacity. Cost Several factors may affect the cost of installing a contraflow lane for emergency evacu- ations along a roadway section: • Construction costs o Gate purchase and installation costs o Cost for tying into existing intelligent transportation system (ITS) infrastructure o Purchase of temporary or portable signs to be used during evacuation o Purchase of temporary barricades to be used during evacuation o Construction of permanent dynamic signing to be used during evacuation

92 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION o Installation of permanent signs for use during evacuation o Installation of permanent fold-out–type signs • Ongoing costs o Police officers to direct traffic o Implementation of temporary message boards to be used during evacuation o Implementation of gates or barricades o Implementation of temporary or portable signing o Service and maintenance for gates, ITS, and other devices Contraflow Lanes for Work Zones Description and Objective A contraflow lane is a lane that has been “borrowed” from one direction of travel to add capacity to the other direction of travel. Contraflow lanes are most often used when either an increase in demand or decrease in capacity necessitates finding ad- ditional capacity to serve vehicles in that direction. Common applications are during peak hours when demand is highly directional, during emergency evacuations, and during work zones. The first application is used in circumstances of recurrent conges- tion due to daily demand fluctuations and is outside the scope of this research. Contra- flow lanes in emergency evacuations are discussed in the previous section. Contraflow lanes as a nonrecurrent congestion treatment during work zones are considered here. Contraflow lanes can improve roadway operations during work zone and mainte- nance activities, as illustrated in Figures 4.2 and 4.3, by providing additional capacity for the direction of travel with reduced capacity due to a closed shoulder or lane. How- ever, consideration must be given to capacity and demand in the opposing direction of traffic before borrowing a lane, because reducing capacity in the opposite direction may have significant negative effects if demand levels are sufficiently high. Typical Applications Contraflow lanes may be used for routine temporary work zones or nonroutine semi- permanent work zones. Contraflow lanes are frequently used on both arterials and freeways. On freeways, vehicles in one direction of travel are often routed through a median or barrier to a lane on the other side. This lane is separated from the adjacent lanes, which are still accommodating traffic flowing in the opposite direction, by some combination of double yellow striping, delineators, or a barrier. On arterials, contra- flow lanes may be temporary in nature, perhaps to be used only during certain hours of the day, and are frequently delineated with only cones and signing. On freeways, contraflow lanes are often used in extended, long-term work zones on divided facilities, when one direction of travel is closed for construction or mainte- nance (see Figure 4.4). In these applications, traffic is rerouted from the original travel lanes across temporary median crossovers to lanes in the opposite direction of travel. Depending on the number of lanes, lane width, facility speed, and duration of the work zone, traffic in the contraflow lane may be separated from adjacent opposing traffic

93 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION with striping changes, concrete barriers, or flexible delineators. Additional pavement may be required to route traffic through the median to the contraflow lane. Figure 4.4 illustrates a typical median crossover. On arterials without a median or barrier, signing and cones are often used to temporarily shift traffic in one direction around the work zone and over one lane. For longer-term work zones, the treatment may include changes in striping. Design Criteria The following are criteria for closing one side of a divided roadway and operating two- way traffic on the opposite side: • On a four-lane divided highway, traffic volumes in both directions must each be ac- commodated in one lane. On freeways with six lanes or more, the number of lanes available for each direction of traffic after the contraflow lane is implemented must adequately accommodate the demand. This would imply that volumes should be below 1,600 vehicles per hour per available lane. • No more than one or two interchanges should be included in the work area. • Work operations are difficult to accomplish in a manner that allows some traffic to continue using the direction of freeway where the work zone will be located. That is, consideration should first be given to staging the work zone in a way that keeps one or more lanes open in the primary direction. The primary advantages of using contraflow lanes include the following: • Work zone duration may be shortened by allowing full lane closures and routing traffic to lanes typically used for the other direction of travel. • Traffic can be shifted away from work zones that are very near or encroach on travel lanes so that drivers are not exposed to drop-offs or other hazards created by the construction operations. Figure 4.2. Traffic diverted across median. Source: Federal Highway Administration website.

94 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Traffic can be shifted away from work zones to provide highway workers with more clearance from traffic. The basic disadvantages of the contraflow strategy are as follows: • There is an increased risk of serious head-on collisions when traffic normally sepa- rated by a barrier or median is now flowing in adjacent lanes. Positive separation of the two directions must be considered, especially on high-speed roadways. • On freeway applications with unpaved medians, crossovers must be constructed at the start and end of the contraflow lane and additional treatments may be needed to handle traffic at interchanges. • Shoulders may need to be improved to accommodate crossover traffic, especially if shoulder width is used as part of the contraflow lane. Temporary attenuators may be needed where there are bridge rail or guardrail ends exposed to traffic. Figure 4.3. Median crossover at a ramp for use in contraflow lane for a work zone. Source: Manual on Uniform Traffic Control Devices (FHWA 2009).

95 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Impact of Work Zone A contraflow lane may be used during a work zone to offset the reduction of avail- able lanes in the direction affected by the work zone. For example, if a work zone is planned that will block two lanes, the contraflow lane may borrow one lane from the opposing direction for the duration of the work zone, thereby offsetting the negative impact of the work zone by 50%. Attention must be paid to the demands and capaci- ties of both directions of travel to ensure that operations in the direction from which lanes are borrowed are not extensively degraded. Figure 4.4. Typical median crossover on a divided highway. Source: Manual on Uniform Traffic Control Devices (FHWA 2009).

96 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION By providing additional capacity during a work zone event, contraflow lanes can improve roadway operations by reducing delay and improving reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of contraflow lanes at reducing non- recurrent congestion due to work zones: • The number of lanes to be shifted • Traffic volume in the direction of the proposed contraflow lane • Roadway capacity in the direction of the proposed contraflow lane • Traffic volume in the opposing direction of the proposed contraflow lane • Roadway capacity in the opposing direction of the proposed contraflow lane Contraflow lanes for short-term work zones will be the most cost-effective when volume-to-capacity ratios are high in the direction of the proposed contraflow lane and low in the opposing direction. For long-term work zones, the direction of peak flow is expected to change throughout the day, and this consideration may not be relevant. Cost Factors that may affect the cost of a contraflow lane for use in a work zone include the following: • Length of proposed contraflow lane • Need for modular concrete barriers to separate opposing directions of travel • Availability of temporary concrete barriers (i.e., will barriers be required for the construction project anyway, and therefore on site regardless of whether contra- flow lanes are used?) • Need for temporary pavement (e.g., to allow vehicles to move from one side of a divided highway to the other for contraflow operation) • Contraflow signing • Temporary barricades • Temporary pavement construction • Temporary pavement marking (on existing and temporary pavement) HOV Lanes and HOT Lanes Description and Objective A high-occupancy vehicle (HOV) lane, often referred to as a carpool lane, is a lane dedicated to those traveling with at least one passenger in addition to the driver in the vehicle; HOV lanes are also often used by buses. HOV lanes are designated with sign- ing and pavement marking or, in some cases, a physical barrier. They may operate as HOVs permanently or only during peak hours. Typically, HOV lanes are patrolled by law enforcement, and violators are ticketed.

97 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION High-occupancy toll (HOT) lanes are similar to HOV lanes, but instead of being exclusively for vehicles with passengers, they are also open to motorists without pas- sengers but who are willing to pay a toll to use them. Hours of tolling operation may be based on the level of congestion or prevailing speed in the HOT lane or in the adja- cent single-occupant vehicle lanes, or on time of day. The objective of this treatment is to reduce nonrecurrent congestion by temporar- ily opening an HOV (or HOT) lane to all traffic. During an incident that causes one or more lanes to be blocked, the HOV lane is opened to all traffic, increasing the roadway capacity and allowing the queue to dissipate. Once the incident is cleared and traffic flow returns to normal, the HOV lane can resume normal operation, as illustrated in Figures 4.5 and 4.6. Figure 4.5. HOV lane. Source: Neudorff et al. (2003). Figure 4.6. HOV lane in Atlanta, Georgia. Source: Transportation Conformity Brochure (FHWA 2010).

98 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Typical Applications HOV–HOT lanes are widely used around the country to encourage carpooling and transit ridership along heavy commuter routes. They are often retrofitted on existing roadway cross sections by using some or all of the shoulder and striping narrower general-purpose lanes. In circumstances of strong directional traffic, a contraflow lane may be used as an HOV or bus lane. This lane may be separated from opposing traffic by a movable barrier or flexible delineators that are removed during those times of day when the contraflow lane is not in use. Additional lanes are sometimes built in the me- dian of a divided freeway as dedicated HOV or HOT lanes. These lanes may be revers- ible, operating in the peak direction by time of day, or they may serve both directions in two-way operation. HOV–HOT lanes may be separated from general-purpose lanes with concrete barriers, double-line striping, single-line solid striping, a buffer area or shoulder, or other special delineation. HOV lanes may have restricted ingress and egress or may allow vehicles to enter and exit at any point along the lane. Most HOT lanes have restricted access so that single-occupant vehicles that are required to pay the toll for using the lane can be accounted for at tolling locations. In some applications, HOV–HOT lanes have dedicated ramps and fly-overs to connect the HOV–HOT lane on one route to the HOV–HOT lane on another route, eliminating the need for HOV– HOT lane users to merge with general traffic to exit and enter. Design Criteria Four distinct facility types can be used in the design of HOV lanes: • Concurrent-flow HOV lanes • Contraflow HOV lanes • Two-way barrier-separated HOV facilities • HOV lanes in separate rights-of-way A concurrent-flow HOV lane may be separated from adjacent traffic by a narrow buffer area. The buffer area is delineated by a solid white edgeline on the inside and a solid double yellow line on the outside. Both pavement markings and static signs are used to inform motorists of the intent of the inside lane. A contraflow HOV lane uses the leftmost lane in the low-volume direction to accommodate HOV traffic in the opposing direction. The lane can be separated from the rest of the facility by removable flexible delineators. Limiting a contraflow lane to buses and vanpools ensures that the drivers are trained to safely navigate while travel- ing in the opposing direction of adjacent traffic. Removable delineators allow the lane to be switched from normal operation to contraflow and back by time of day. In some cases, a separate HOV facility is constructed between the opposing lanes of freeway traffic. The HOV lanes operate in one direction during a.m. peak periods and the opposite direction during p.m. periods. The HOV facility is separated from general traffic by permanent concrete barriers or by shoulders and grass medians. The fourth type of HOV facility, in which the lanes are in separate rights-of-way from the general traffic, is often for dedicated bus routes or other rapid transit lines. Because these facilities do not interact with the freeway in the same way as the other

99 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION three types of facilities (i.e., they are not designed to facilitate vehicles moving back and forth between the general-purpose lanes and the HOV lanes), they probably have less potential to reduce nonrecurrent congestion. Each highway agency may have its own design standards for each of the four types of HOV facilities, but the American Association of State Highway and Transporta- tion Officials’ Guide for High-Occupancy Vehicle Facilities (AASHTO 2004) provides desired and minimum cross-section designs, as well as design guidance for ramps, fly- overs, tapers, enforcement areas, transitions, and bus stops. When choosing an HOV facility design, thought should be given to the ease with which reversible lanes can be flushed and redirected, the frequency of ingress and egress points along barrier-separated facilities, the ease with which barriers or delinea- tors can be moved, the width of shoulders and how easily they could be used as lanes, and other factors that may allow HOV lanes to be converted to general-purpose lanes in the event of severe nonrecurrent congestion. How Treatment Reduces Nonrecurrent Congestion Increases Capacity of Segment During an Incident In many cases, there is not sufficient HOV demand for an HOV lane to operate at capacity. Even when traffic operations break down and speeds decrease in the normal lanes, the HOV lanes may continue to operate well below capacity. When a major incident takes place and leads to significant queuing on the freeway, the HOV lane may be opened to all traffic. Vehicles will then spread over all lanes equally, resulting in an increase in capacity of the roadway segment. After the incident is cleared and the queue dissipates, the HOV lane may again be closed to non-HOV traffic. Opening an HOV lane to all traffic has a negative impact on the HOV traffic, because the advantage of carpooling or riding the bus is eliminated during the period when the lane is open to all traffic. For this reason, such action is recommended in response to major incidents only. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of HOV–HOT lanes at reducing non- recurrent congestion: • Demand-to-capacity (d/c) ratio of the HOV lane • Type of separation between the HOV lane and normal lanes (pavement marking only or barrier) • Infrastructure for notifying drivers of lane opening (e.g., dynamic message boards) Cost Factors that may affect the cost of opening an HOV lane to general traffic include the following: • Improvements to infrastructure for notifying drivers of lane opening • Loss of toll revenue during period when HOT lane is open to all traffic for free

100 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Dynamic message board construction and operation costs • Costs of additional staff to monitor roadway conditions and initiate opening of HOV lane to all traffic In some cases, this treatment may be implemented for a negligible cost, particularly along freeways with existing traffic management centers. If infrastructure improve- ments are needed, costs may range from thousands to tens of thousands of dollars. Dual Facilities Description and Objective Dual facilities provide additional freeway managed lanes, not simply set aside by strip- ing or barriers, but a second set of mainline facilities that are separate, parallel, and adjacent to the primary freeway. A dual–dual configuration may allow trucks and pas- senger cars to use separate facilities within the same freeway envelope. The inner road- way may be dedicated to passenger cars and light vehicles, while the outer roadway is available to all vehicles, including heavier trucks and buses. Each separate roadway has its own entrance and exit ramps. The purpose of a dual facility is to separate the traffic streams between disparate types of vehicles that would normally be combined in mixed-flow lanes. Objectives for this separation include enhanced traffic safety, opportunities for alternative treatments of different user types, and reduced congestion. Typical Applications The New Jersey Turnpike is the primary example of a dual facility freeway. It has over 30 mi of inner lanes (for passenger cars) and outer lanes (for truck, bus, and car traffic) within the same right-of-way. For most of this length (23 mi), the inner and outer roadways have three lanes in each direction. On one 10-mi section, the outer roadway has two lanes per direction, and the inner roadway has three lanes per direc- tion. Each roadway has 12-ft lanes and shoulders, and the inner and outer roadways are barrier separated. Figure 4.7 presents a photo of interchange bypass lanes in Los Angeles, California. By using gates or signage, operators can limit access to a particular roadway as needed to manage typical demand peaks, as well as to respond to incidents and other nonrecurring events. The result is a roadway that operates efficiently because turbu- lence in the traffic flow is minimized. When the New Jersey Turnpike experiences an incident that closes the outer road- way and traffic is diverted into the inner roadway, lane restrictions are employed on the inner roadway. Regulatory signs reading “NO TRUCKS OR BUSES IN LEFT LANE” are displayed. Passenger cars are also restricted by the following regulatory sign message: “CARS USE LEFT LANE FOR PASSING ONLY.”

101 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria When evaluating a route to determine whether a dual facility will be good fit, the fol- lowing items should be considered: • Merging zones • Exit and entrance ramps • Interchange ramps • Access to local interchanges • Results of heavy-truck flow survey • Results of toll analysis • Results of demand analysis • Overcoming problematic use of current ramps (may not be suitable for heavy trucks) Dual facilities offer a unique opportunity for handling nonrecurrent congestion. The major benefit to motorists is having a supplemental adjacent freeway available on which to divert. On a typical freeway, when nonrecurrent congestion occurs as the result of an incident that blocks a lane, the impact to delay can be significant, with some of the affected traffic diverting onto the surface street network. However, when this type of incident occurs on a dual-facility freeway, the first alternative route avail- able is the adjacent, parallel, controlled-access roadway. For many motorists, this is a more favorable alternative than the surface street network. This is particularly true during hours when the freeway is not operating at capacity. In the case of a full road closure, when all lanes in one direction on either the inner or outer roadways are closed, the value of a dual facility as a diversion route is significant. Figure 4.7. Interchange bypass lanes in Los Angeles, California. Source: Neudorff et al. (2003).

102 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents The safety benefits of separating auto and truck traffic may be one of the most important benefits of truck lanes (Fischer et al. 2003). By reducing the frequency of crashes, this treatment also reduces the nonrecurrent congestion associated with these crashes. Reducing the number of times that a lane is blocked by a crash- involved vehicle reduces congestion and improves reliability. Increases Base Capacity of a Segment The operational benefits of dual facilities are expected to result from separating small vehicles from large vehicles. These different vehicle fleets have different operating char- acteristics, such as acceleration–deceleration rates, visibility, and conspicuity. Separat- ing these vehicle classes into separate facilities enhances the uniformity of operations within each roadway and thus improves traffic operations. This increase in base capacity causes a reduction in the d/c ratio, a key factor in nonrecurrent congestion. The lower the d/c, the less negative impact a lane-blocking incident has on roadway operations, and the greater the reliability of the roadway segment. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of dual facilities in reducing nonrecurrent congestion include the following: • Demand volumes (all traffic) • Proportion of all traffic made up of heavy vehicles • Crash frequency • Capability of warning motorists of congested conditions in advance so they can divert onto the noncongested facility Dual facilities will be most effective at reducing nonrecurrent congestion along freeways with high-demand volumes, a high proportion of heavy-vehicle traffic, and high crash rates. Cost Construction of dual facilities is generally very expensive, potentially including the purchase of right-of-way, extensive design work, as well as roadway and bridge con- struction costs. Reversible Lanes Description and Objective Reversible lanes are lanes on which traffic flows in one direction during certain times of day and in the opposite direction during other times of day. Reversible lanes can be used to add capacity in the direction of the dominant flow for general-purpose traffic or for limited traffic such as buses or HOVs.

103 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The objective of reversible lanes is to address unbalanced flow, which typically occurs during the morning and evening peak travel times on routes heavily traveled by metropolitan commuters. The purpose is to match directional lane capacity to the proportion of directional traffic flow. Rather than adding capacity to both directions of travel, which do not require the additional capacity at the same time, fewer lanes can be constructed and shared between the two directions, accommodating each direc- tion as needed. Because reversible lanes may be opened to a given direction of travel in response to nonrecurrent events (e.g., lane-blocking crash, work zone), they are a useful treatment for reducing nonrecurrent congestion. Typical Applications Reversible lanes are used on both freeways and arterials. On freeways, the most com- mon application is one or more lanes located in the median. These lanes may be sepa- rated from the general-purpose lanes by a barrier or median. They have limited ingress and egress points that can be directionally switched by time of day, so that the lanes are open to one direction of traffic during the morning peak and another in the evening peak. On arterials, these lanes are often not separated from the adjacent lanes but are indicated by special striping and signs. Changeable lane-use signs may be placed over- head at certain locations along the reversible lanes to indicate the direction in which the lanes are operating at any given time. Arterial reversible lanes are sometimes used to handle daily recurrent congestion, but they are also often used to accommodate traffic for special events near sports stadiums (see Figure 4.8), arenas, and other venues. Figure 4.8. Reversible lane used for clearing stadium traffic in Baltimore, Maryland.

104 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Design Criteria The AASHTO Green Book (2011) provides design guidance for HOV reversible con- figurations. These elements include transition areas (between the general-purpose lanes and the reversible lanes), entry and exit points at ramps and midsegment, and cross-section width. NCHRP Synthesis of Highway Practice 340: Convertible Roadways and Lanes: A Synthesis of Highway Practice (Wolshon and Lambert 2004) suggests that the design criteria for many characteristics of reversible roadways are identical to the standards set forth in the Federal Highway Administration’s (FHWA) Manual on Uniform Traffic Control Devices (MUTCD) (2009) and the Green Book (AASHTO 2011). This overlap in standards may have occurred because many reversible lanes in use were originally designed for conventional use and have since been retrofitted as reversible lanes. However, the report notes that on newer facilities that are being constructed as reversible lanes, and on freeways where physical lane separation is necessary, special design treatments may be necessary. How Treatment Reduces Nonrecurrent Congestion Increases Capacity of Segment During an Incident Reversible lanes can be opened during nonpeak periods in response to nonrecurrent lane-blocking events such as a major crash or a temporary work zone. In these cases, the decreased capacity caused by the nonrecurrent event is mitigated by the additional capacity provided by the reversible lane. By increasing capacity at these times, this treatment can reduce nonrecurrent congestion and improve reliability. Factors Influencing Treatment Effectiveness Very few evaluations have measured the operational or safety effectiveness of revers- ible lane facilities. Those studies that have addressed operational effectiveness typically measured operational effectiveness either in terms of traffic volume, travel time, and/or travel speed increases after the implementation of the reversible lanes. For example, an evaluation of a reversible lane installed in Dearborn, Michigan, reported increases in total traffic volumes between 3% and 7% (Wolshon and Lambert 2004). On the same facility, travel time was reduced by approximately 16%, and travel speeds increased approximately 21%. The effectiveness of reversible lanes at reducing nonrecurrent congestion will be based on the following: • Number of times that a reversible lane will be deployed in response to a conges- tion-causing incident • Traffic demand levels during nonpeak times (because, presumably, the reversible lane will already be open in the given direction during peak times and cannot be deployed in response to a congestion-causing event) • Percentage capacity added to the roadway by the opening of the reversible lane • Duration of time that passes between a congestion-causing event occurring and the opening of the reversible lane

105 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Implementation of reversible lanes is widely considered one of the most cost-effective methods of increasing peak period capacity along existing streets. Several direct costs are associated with the maintenance and operations of reversible lanes: • Pavement construction (if applicable) • Median reconstruction • Law enforcement to prevent violations of lane-use restrictions • Personnel to set up and remove traffic control devices • Staff to operate and strategically manage the system Work-Zone Express Lanes Description and Objective This treatment involves the construction of express lanes that bypass a freeway work zone without any additional entry or exit points. The targeted application of this treat- ment is in freeway environments in which entire directions of travel must be closed to allow for new roadway construction and improvements. Because of the limited access of this treatment, it is naturally complemented with exterior frontage roads to handle traffic that has entry or exit needs within the work zone area. The objective of work-zone express lanes is to allow traffic to move through a work zone area without being inhibited by an arterial street environment. When extensive construction exists along a freeway, traffic must either be diverted to local streets or directed along specially designated express lanes. For vehicles that have no local demand within the work zone area, an express lane provides a safe and efficient route through what might otherwise become a congested area. This congestion can be due, in part, to turbulence resulting from excessive weaving and turning maneuvers under high volumes. How Treatment Reduces Nonrecurrent Congestion Increases Capacity of Segment During a Work Zone Work-zone express lanes increase the capacity of a segment by eliminating the weav- ing and turbulence created by entering and exiting vehicles. The work zone will likely cause reduced capacity for the duration of the construction project, and work-zone express lanes can be an effective tool in mitigating the capacity decrease caused by the work zone. By increasing capacity during the period of the work zone, this treatment can reduce the amount of nonrecurrent congestion caused by the work zone, thus im- proving reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of work-zone express lanes at reducing nonrecurrent congestion: • Demand volumes • Duration of work zone

106 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Capacity decrease expected due to the work zone (e.g., one lane of a three-lane cross section is closed) • Capacity of the express lanes Cost The cost of installing a work-zone express lane will vary based on the length of road- way to be constructed, the availability of right-of-way, and the extent of traffic control needed. TRAFFIC SIGNALS AND TRAFFIC CONTROL Traffic Signal Preemption Description and Objective Traffic signal preemption is a system that allows the normal operation of traffic signals to be interrupted. Typically, traffic signal preemption systems give priority to emer- gency vehicles by changing traffic signals in the path of the approaching emergency vehicles to green (or in some cases, flashing green) and stopping cross traffic. The ob- jective of traffic signal preemption systems is to reduce the response time of emergency vehicles. Preemption systems also reduce the likelihood of a secondary incident involv- ing an emergency vehicle en route to an incident. Traffic signal preemption technologies employed today include light-, infrared-, sound-, and radio-based emitter–detector systems. Stakeholders need to gather infor- mation and consider key operational features and interoperability requirements as they plan deployments of such systems. The FHWA Traffic Signal Timing Manual (Koonce et al. 2008) outlines several types of technologies available to vehicles requesting pre- emption, including light (strobe), sound (siren), pavement loops, radio transmission, and push buttons. Table 4.1 summarizes technical considerations for the various pre- emption technologies. TABLE 4.1. SUMMARY OF TECHNICAL CONSIDERATIONS OF VARIOUS TRAFFIC SIGNAL PREEMPTION OPTIONS Technology Consideration Strobe Activated Siren Activated Radio Activated Dedicated vehicle emitter required Yes No Yes Susceptible to electronic noise interference No No Yes Clear line of sight required Yes No No Affected by weather Yes No No Possible preemption of other approaches No Yes Yes Source: FHWA (2005).

107 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION A recent FHWA report, Traffic Signal Preemption for Emergency Vehicles: A Cross-Cutting Study (2005), identifies issues associated with emergency vehicle pre- emption. By using data from three jurisdictions that operate traffic signal preemption systems, the report identifies the following benefits of the systems: • Reduction in response times. • Reduction in the number of emergency vehicle crashes. • Ability to achieve the same response times with fewer fire and rescue and emer- gency medical services stations than would normally be required as a result of the traffic signal preemption system (this benefit provides significant cost savings). How Treatment Reduces Nonrecurrent Congestion Reduces Emergency Response Time Emergency vehicle signal preemption systems allow emergency personnel to navigate through an intersection more quickly, thus reducing their response time to an incident scene. This reduces the period of time during which disabled vehicles remain in the roadway or on the roadway shoulder. By reducing the lane-blocking time of an inci- dent, the nonrecurrent congestion associated with that incident is reduced, and reli- ability is improved. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of signal preemption systems at reduc- ing nonrecurrent congestion: • Frequency of system use • Average travel time savings for emergency personnel using the system • Frequency of crashes involving emergency personnel en route to another incident scene Cost Factors that may affect the cost of constructing and operating a traffic preemption system include the following: • Technology type (light or sound based) • Number of units purchased • Intersection cost variables o Availability of power on the mast arm o The need to run new power or communication cables (or both) through exist- ing conduits o Availability of suitable detector placement locations • Existing emergency vehicle provisions for housing the power supply and emitter

108 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Queue-Jump and Bypass Lanes Description and Objective Queue-jump lanes are built at congested intersections or ramps to allow transit vehicles, HOVs, or toll-paying vehicles to bypass the queue at the signal or ramp meter. One example of a queue-jump lane is a short lane provided at the signal approach and con- tinued for a short distance beyond the signal. Typically, this lane is dedicated to transit or HOVs, and the lane is controlled by a dedicated signal that allows the lane to clear before releasing adjacent traffic. The working mechanisms of queue-jump lanes are traffic signal priority devices, which connect the traffic signal with the bus presence. Bus bypass lanes are built on roadways where considerable bus traffic and con- siderable mixed traffic exists. These lanes are used to bypass the congestion of mixed traffic and minimize the mixed traffic and bus interaction. The primary objective of queue-jump lanes and bypass lanes is to allow higher- capacity vehicles to advance to the front of the queue at a signal, reducing the delay caused by the signal and improving the operational efficiency of the transit system. However, this objective relates exclusively to recurrent congestion. The objectives of a queue-jump or bypass lane at reducing nonrecurrent congestion include (1) reducing emergency response time and (2) providing a path for traffic around a lane-blocking incident. Figures 4.9 and 4.10 present examples of queue-jump lanes. Figure 4.9. Queue-jump lane on a metered ramp in Minnesota.

109 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Typical Applications Queue-jump lanes are installed at intersections. The queue-jump lane can be a right- turn-only lane that permits through movements for buses only. They can also be in- stalled between right-turn and through lanes. In Minneapolis–St. Paul, Minnesota, queue-jump lanes are provided at several metered ramps to allow carpools, buses, and motorcycles to bypass any queues formed at the ramp meter. Bus bypass lanes are roadway lanes that are dedicated to bus and metro use only. Bus bypass lanes could be installed on the rightmost lane or shoulder of high-volume arterials and freeways. In Kansas City, Missouri, bus bypass lanes are installed along the curbside edge. This arrangement eliminates roadside parking, but it allows passen- ger embarking and disembarking to continue without affecting roadway traffic. Design Criteria Installations of queue-jump lanes typically involve modifying signal timing plans at the intersections. Design considerations include signing, lane and shoulder widths, merging area (where queued vehicles and bypassing vehicles have to merge), and sight distance. Three types of systems are used for traffic signal priority: optical, wayside reader, and “smart loop.” TCRP Report 19 (Fitzpatrick et al. 1996) indicates that queue-jumper bus bays may be considered on arterial street intersections when the following factors are present: • High-frequency bus routes have an average headway of 15 min or less. • Traffic volumes exceed 250 vehicles per hour in the curb lane during the peak hour. Figure 4.10. Queue-jump lane with advance stop bar. Source: Levinson et al. (2003).

110 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • The intersection operates at Level of Service D or worse. • Lane acquisitions are feasible and costs are affordable. • An exclusive bus lane, in addition to the right-turn lane, should be considered when right-turn volumes exceed 400 vehicles per hour during the peak hour. Multiple variations of bus bypass lanes are found on mainline freeways and ramps. One variation is the use of a shoulder or added constructed lane on the right side of the road that is only used by buses. This lane ends with a lane drop where the bus bypass lane merges with the rightmost lane of roadway. The lane is divided into three sections. The initial section is the entrance merge, where buses can enter the bus bypass lane. The second section is a normal queue length, a length of roadway where the bus lane is a uniform width running in alignment with the roadway. The third section of the bus bypass lane is the drop section, where the roadway merges left into the rightmost lane over a distance. Ample markings and signage should occur just before and within this section of roadway to notify both mainline traffic and bus traffic of the lane merge. The second type of bus bypass lane is a convergence ramp. The convergence ramp allows buses to bypass congested traffic by using an additional travel lane on the entrance ramp. The convergence ramp is composed of three sections. The initial sec- tion is the entrance merge, where buses can enter the bus bypass lane. The second section is a normal queue length, a length of roadway where the bus lane is a uniform width running in alignment with the roadway. The third section of the convergence ramp is the convergence section, where the roadway expands into an additional lane with the ramp lane and bus lane. Ample markings and signage should occur just before and within this section of roadway to notify both mainline traffic and bus traffic of the lane merge. How Treatment Reduces Nonrecurrent Congestion Reduces Emergency Response Time Queue-jump or bypass lanes can be used by emergency personnel to bypass queued vehicles while en route to an incident scene. Reducing emergency response time re- duces the amount of time that an incident-involved vehicle remains on the roadway blocking a lane. Reducing this lane-blocking time reduces the congestion associated with these incidents, leading to improved reliability. Increases Capacity of Segment During an Incident In the event that an incident occurs on an intersection approach and does not block access to the queue-jump (or bypass) lane, the lane may provide traffic access around the incident until the incident can be cleared. By increasing capacity during a lane- blocking incident, this treatment can reduce nonrecurrent congestion and improve reliability. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of queue-jump and bypass lanes at re- ducing nonrecurrent congestion:

111 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Length of queue-jump or bypass lane (longer lanes will be easier for emergency response personnel to access when queues exist) • Traffic demand levels • Incident frequency • Local policies encouraging or discouraging general traffic to bypass an incident by using the queue-jump lane Roadways with high frequencies of lane-blocking incidents, high-demand levels, and local policies encouraging general traffic to bypass an incident using the queue- jump lane will be most positively affected by this treatment. Queue-jump and bypass lane treatments are also more effective when the bypass lane is sufficiently long to bypass the general traffic queue and the right-turn volume in the bypass lane is rela- tively low. Cost Factors that may affect the cost of constructing queue-jump or bypass lanes include the following: • Need for additional right-of-way • New pavement construction costs • Traffic signal hardware and materials needed • Modification of signal timing • New signing • Pavement markings • Adjustments to roadway lighting or other utilities Traffic Signal Improvements Description and Objective Traffic signal improvements encompass a wide range of specific actions or treatments at signalized intersections. These include improvements to the physical signal system itself, such as the following: • Installing new signals • Upgrading span-wire signals to permanent signals on poles and mast arms • Adding supplementary signal heads to increase signal head visibility • Increasing signal head size from 8 to 12 in. • Changing a three-head signal to a four- or five-head signal to allow for protected left turns • Moving signal poles farther from the roadway to minimize the likelihood of vehicles knocking them down

112 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Upgrading to signal controllers that can handle closed loop, responsive, or adap- tive signal timing plans • Installing or upgrading vehicle detection at intersections (inductor loops or cameras) Treatments may also include improvements to the signal timing plan, such as the following: • Improving clearance intervals to maximize efficiency while taking safety concerns into consideration • Changing phasing to add or remove protected left turns • Implementing or improving coordination plans along a corridor • Implementing responsive or adaptive signal timing systems • Adjusting timing elements such as minimum green, maximum green, gap time, and overlaps • Providing pedestrian phases with pedestrian push buttons and signal heads In addition, automated enforcement for red light running or speed violations are signal-related improvements that may be implemented to reduce incidents caused or related to these violations. The primary objective of any signal timing plan is to minimize conflicts at an inter- section. Signal timing improvements and improvements to the signal hardware and infrastructure are often aimed at either improving the efficiency of the intersection (or corridor) or improving safety for vehicles, bicyclists, and pedestrians at the intersec- tion. Figure 4.11 presents an example of a signal improvement. Figure 4.11. Lane-aligned signal heads. Source: Rodegerdts et al. (2004).

113 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Crash Frequency Improvements to traffic signals may lead to a reduction in total crashes. By eliminating crashes, this treatment eliminates the congestion associated with these crashes, thereby improving reliability. FHWA and the Institute of Transportation Engineers published a document in Sep- tember 2007 related to traffic signal timing and safety. The authors of this document found that “[t]he following changes may decrease crashes: • Signal retiming, phasing, and cycle improvements; • Review and assurance of adequacy of yellow change interval/all-red clearance; interval for safer travel through the intersection; • Use of longer visors, louvers, backplates, and reflective borders; • Installation of 12-in. signal lenses; • Installation of additional signal heads for increased visibility; • Provision of advance detection on the approaches so that vehicles are not in the dilemma zone when the signal turns yellow; • Repositioning of signals to overhead (mast arm) instead of pedestal-mounted; • Use of double red signal displays; and • Removal of signals from late-night/early-morning programmed flash.” Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of traffic signalization improvements at reducing nonrecurrent congestion: • Signal-related intersection delay before improvements • Signal spacing through the corridor • Vehicle detection before improvements • Number of pedestrians using the intersection • Number of special users near the intersection (children, bicyclists, senior citizens) • Traffic demand volumes • Intersection geometry and lane configuration • Crash pattern and frequency before improvement • Existing clearance intervals • Visibility of signal heads Traffic signalization improvements will provide the most benefit in locations with high crash rates and where demand levels are often near capacity.

114 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Signal timing improvements often have high benefit–cost ratios because the cost may be little more than a few hours of staff time to develop and implement an improved plan, although the benefit could be the reduction of several severe crashes. Improve- ments to vehicle detection (cameras or induction loops); signal controller upgrades; and replacement or addition of signal heads, backplates, and other infrastructure can cost from hundreds of dollars to tens of thousands of dollars per intersection. Signal Timing Systems Description and Objective Certain signal timing systems are capable of changing timing plans at an intersection or along a corridor as traffic conditions change. These systems require top bar detec- tion, and in many cases, upstream and midblock detection, as well. Induction loops in the pavement, cameras, or radar may be used to detect vehicle presence, density, speed, and/or travel time, and this information can be conveyed to the signal con- troller, which determines the most appropriate signal timing. Responsive signal timing systems such as closed loop systems use information from the detectors along a signalized corridor to choose the most appropriate signal timing plan from a library of plans. A traffic engineer develops a library of plans before the system is implemented, and each plan in the library is assigned certain threshold limits in the traffic characteristics. These thresholds are determined by observing the traffic flow under various conditions (e.g., off peak, evening peak, at the release of a sporting event) and by the engineer’s judgment. For example, the engineer may observe that during the holiday shopping season, the ramps at the interchange near the shop- ping mall back up onto the highway because the signal timing at the ramp does not permit enough vehicles to move off the ramp during each cycle. A loop detector near the end of the ramp can be used to detect the presence of vehicles being stored there, and when the presence reaches a certain threshold (perhaps cars are detected for 50% of the time or more), the controller will chose a plan that allows more green time to the phases that serve the ramp. This new timing plan will run until the detectors indicate that vehicle presence has fallen below the assigned threshold. In most closed loop systems, the information gathered from the detectors is fed to a master controller that chooses the timing plan that will be run. The master then tells all the other controllers in the corridor which plan to implement locally and provides the proper offset to each controller so that the corridor will continue to operate in coordination no matter which plan is chosen. Adaptive signal timing systems use vehicle detection at the intersection approach to assess real-time demand at each leg of the intersection. They then use an algorithm to assign priority to each signal phase according to the number of cars that are queued. The order of signal phases and the green time allocated to each phase are based on the priority assignment given by the algorithm. In an adaptive signal timing system, real-time data at the intersection and along the corridor are used to determine the most efficient signal timing plan, and this plan changes with changing conditions.

115 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Adaptive systems do not necessarily have defined signal timing plans with a set cycle length or offsets. They do not have to move sequentially through a set of phases, because the control- ler can skip phases, or move back to a previous phase, to best minimize delay. The objective of responsive and adaptive signal systems is to make each intersection, and the corridor as a whole, more efficient by using real-time information about vehicle demand and traffic flow. As green time is allocated more efficiently, vehicle delay and congestion are reduced. Fig- ure 4.12 illustrates an example of a signal timing system. How Treatment Reduces Nonrecurrent Congestion Increases Capacity During an Incident Responsive and adaptive signal timings can adjust green time allocation in response to an incident. For example, if a westbound through lane is blocked by a crash-involved vehicle, the westbound through movement capacity will be greatly diminished. Re- sponsive or adaptive signal timing can react to this event by providing a longer green time, effectively increasing capacity for this movement. This capacity increase results in reduced congestion and delay along this segment. Providing additional green time to one phase necessarily takes time from another phase, leading to increased delay in the latter direction. Overall, however, the system should see a net benefit despite the increased congestion in the nonincident direction. Factors Influencing Treatment Effectiveness The following factors are important in determining the effectiveness of signal timing systems for reducing nonrecurrent congestion: • Type of signal control (responsive or adaptive) • Demand volumes for all movements • Quality of vehicle detection system • Number of crashes Signal timing improvements will provide the most benefit in locations with high crash rates and where demand levels are often near capacity. Figure 4.12. A series of traffic signals controlled by a signal timing system. Source: FHWA (2011).

116 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost The cost of implementing signal timing improvements varies considerably; it is most dependent on the type of controller and detection already present at the intersection. When these are already sufficient, closed loop responsive systems may be implemented for only the cost of the staff time required to develop timing plans and threshold crite- ria. When controllers and cabinets must be replaced or upgraded, and detection must be installed or upgraded, the cost is substantially higher. Ramp Metering and Flow Signals Description and Objective Ramp metering uses traffic control devices to control or meter the flow of vehicles from a freeway ramp onto a freeway mainline. Ramp meters help reduce the poten- tially disruptive impact of the ramp traffic on the heavier mainline freeway traffic. Vehicles are released from the on-ramp at a regular interval, one or two at a time. Ramp meters typically consist of one or more traffic signal heads, as well as associated signs and pavement markings. Geometric considerations on the ramp and approach roadways include ramp width, queue storage space, and turn lanes. Signal timing at a ramp metering location can be either pretimed or traffic responsive. Timing plans can also be based on localized or systemwide factors. When traffic-responsive operations are employed, vehicle detection and communication equipment are needed. For systemwide traffic-responsive installations, the detection and communication equipment can be extensive, collecting traffic flow rate and speed data throughout the system. This information can be input into a ramp metering algorithm to adjust the flow of vehicles entering the mainline from each on-ramp with the goal of pro- viding smooth flow from the ramp without vehicle platooning. As the mainline vol- umes increase, the ramp flows may be decreased to maintain higher system speeds and capacities. If necessary, the ramp flows can also be adjusted to prevent back-ups onto nearby arterials. Ramp metering is used in several major metropolitan areas in the United States, as well as in Europe, Japan, South Africa, New Zealand, and Australia. The objective of ramp metering is to increase the capacity of a freeway segment at an on-ramp junction by introducing on-ramp traffic into the mainline in a steady pattern. Typically, this is done during peak periods when additional capacity is most needed, though ramp meters could also be activated in response to a crash or special event. Photos of ramp meters are shown in Figures 4.13 and 4.14. How Treatment Reduces Nonrecurrent Congestion Increases Capacity During an Incident Ramp meters usually operate during defined peak periods to reduce recurrent conges- tion. However, they can also be used to reduce nonrecurrent congestion if they are activated during a nonrecurrent congestion event. For example, a serious crash may block multiple lanes during a nonpeak period when the ramp meters do not generally run. If the ramp meters are activated, the roadway capacity will be increased, helping to offset the negative effects of the lane block. Once the crash is cleared and traffic normalizes, the ramp meter can be turned back off and normal operations resumed.

117 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Reduces Frequency of Incidents Several sources indicate that ramp metering systems can reduce the number of crashes on a freeway segment (Minnesota Department of Transportation 2006). This reduc- tion is accomplished by smoothing operations at the on-ramp–mainline junction and eliminating platoons of vehicles entering the freeway in rapid succession. By reducing the number of crashes, lane-blocking time is reduced, nonrecurrent congestion is re- duced, and reliability is improved. Factors Influencing Treatment Effectiveness The following factors are important in determining the effectiveness of ramp meters at reducing nonrecurrent congestion: • Percentage capacity increase provided by ramp metering system, which is affected by o Weaving segment length o Platooning of on-ramp traffic when ramp meter is off o Driver behavior • Capability of ITS to recognize a lane-blocking event and activate the ramp meter Cost Factors that may influence the cost of installing a ramp metering system include the following: • Required roadway ramp improvements • Type of system deployed (fixed, local, or system) Figure 4.14. Ramp meter. Source: FHWA (2004). Figure 4.13. Ramp metering installation (right-lane signal).

118 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION • Required improvements to facilitate communication between system parts • Construction costs such as the following o Controller o Signal heads o Detectors o Signing o Pavement markings o Communication infrastructure o Power source Temporary Traffic Signals Description and Objective Temporary traffic signals are provided at intersections for a limited period of time. Often, temporary span-wire signals are installed and used for the duration of a con- struction project. Alternatively, portable traffic signals can be set up on site to be used during a construction project or during an emergency such as a power outage or a damaged controller cabinet. The objective of temporary traffic signals is to increase intersection capacity dur- ing a work zone or an emergency. Without this treatment the intersection would oper- ate as stop-controlled, with greatly reduced capacity. Portable traffic signals can be used in a variety of situations. As with fixed tem- porary signals, portable temporary signals can be used in construction zones. For example, if a section of road is reduced to one lane, with two-way traffic alternating use of that lane, a portable traffic signal could be used (rather than a flagger) to direct oncoming traffic either to stop or to continue through the roadway section. The design of a properly installed temporary traffic control signal system is based on many factors. The Minnesota Department of Transportation’s Guidelines for the Selection of Temporary Lane Control Systems in Work Zones (2006) provides key considerations for the design of a temporary traffic signal: • Type of signal system. Selecting wood pole with span wire, trailer-mounted with overhead signal heads, or portable pedestal ground-mounted systems as appropriate • Location of signal. Considering the visibility of the signal heads and queues that might result from the installation of the signal • Signal timing parameters. Providing signal timing parameters that minimize delay through work zones while maintaining safe clearance intervals • Monitoring and maintenance. Performing routine quality checks to monitor the operations (such as timing parameters, signal head alignment, and power supply) of temporary traffic signals

119 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Temporary signals for bridge construction are necessary for one-lane operations. The use of temporary signals on bridges allows road construction teams to remove flaggers from the field and dedicate synchronized signal timing between signal units. In addition, temporary signals give extra notice to drivers during nighttime operations. How Treatment Reduces Nonrecurrent Congestion Increases Capacity During an Incident Temporary traffic signals increase the capacity of an intersection during a work zone or emergency event that results in reduced congestion and delay along the affected seg- ment. The negative impact of the work zone or emergency is mitigated by the use of the temporary signal, which results in reduced nonrecurrent congestion and improved reliability. Factors Influencing Treatment Effectiveness The following factors are important in determining the effectiveness of temporary signals for reducing nonrecurrent congestion: • Type of use (during work zone or to respond to emergency situations) • Duration of work zone (if applicable) • Number of expected signal-disabling incidents (if applicable) • Demand volumes Temporary signals will be most beneficial at intersections with high-demand volumes during extended work zones or in locations with high numbers of signal- disabling incidents. Cost The cost of conventional temporary traffic signals with wood poles and span wires varies greatly depending on the length of the work zone, intersection geometry, and other work zone characteristics. Variable Speed Limits and Speed Limit Reduction Description and Objective Variable speed limit (VSL) systems, also known as variable speed reduction systems, use sensors to monitor prevailing traffic or weather conditions (or both) and post appro priate, enforceable speed limits on dynamic message signs. VSL deployments can be constructed as permanent installations or as temporary work zone installations. They are generally classified as either a lane management or a speed management sys- tem. They can be applied to freeways and arterials in both rural and urban contexts. The primary objectives of a VSL system are to manage current and future traffic flows, minimize congestion, and improve highway safety. Temporary work zone VSLs are used to adjust vehicular speeds so that uniform traffic flow and safe travel speeds can be achieved through the work zone area. Permanent deployments are often designed to address safety and traffic congestion issues in specific corridors or locations. They tend

120 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION to be used to address weather issues, grade and roadway condition issues, or heavy traffic congestion (both recurrent and nonrecurrent). In congested conditions, VSLs help smooth traffic flows, allowing motorists to pass through the area more quickly and in a uniform manner. This smoothing results from a greater number of drivers traveling in a tighter speed range and a slowing or metering of vehicles entering the congested area. For example, with VSL deployed, drivers both approaching and traveling through a congested area posted at 35 mph may all travel at speeds between 30 and 40 mph, but stop-and-go conditions would have existed with a posted speed of 55 mph. In some situations, VSL also reduces speed differentials and moderates travel speeds, which can prevent incidents that might cause nonrecurrent congestion. Figures 4.15 and 4.16 illustrate examples of VSL implementations. The temporary deployment of VSL in a work zone often consists of trailers with dynamic message signs. The trailers typically contain various communication equip- ment components, as well as vehicle detection and power generation equipment. How Treatment Reduces Nonrecurrent Congestion Increases Capacity of Segment During an Incident VSLs can be activated during nonpeak periods in response to lane-blocking events such as disablements, crashes, and temporary work zones. In these cases, the decreased capacity caused by the lane-blocking event is mitigated by the additional capacity provided by the VSL. By increasing capacity at these times, this treatment can reduce nonrecurrent congestion and improve reliability. Figure 4.15. Variable speed limit sign in Missouri. Figure 4.16. Variable speed limits on M42 in Manchester, United Kingdom. Source: Neudorff et al. (2003).

121 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Reduces Crash Frequency VSLs help smooth traffic speeds, decreasing the speed differential between fast-moving and slow-moving vehicles. This increase in speed uniformity may lead to a decrease in crash frequency. By eliminating crashes, this treatment eliminates the congestion as- sociated with these crashes, thereby improving reliability. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of VSL systems at reducing nonrecurrent congestion include the following: • Traffic demand volumes • Crash frequency before VSL implementation • Type of VSL system (permanent or temporary) • Sophistication of VSL system (automated with internal algorithms for determining speed limit or requiring manual speed limit management) • Traffic management center capability (to activate VSL in response to incidents) VSL systems will provide the most benefit along roadways with high-demand vol- umes during off-peak time periods, when crashes are relatively frequent. Cost The cost of VSL implementations depends on the length of the corridor to which they are applied, the number of signs needed, the type of sign supports needed, and the sophistication of the detectors and algorithms used. TECHNOLOGY Electronic Toll Collection Description and Objective Originally developed as a solution to systematic recurrent congestion on tolled high- ways, electronic toll collection has also proved useful in reducing nonrecurrent con- gestion in multiple ways. This technology uses communications equipment installed in vehicles and along a roadway segment to collect tolls instantaneously and automati- cally, thus removing the need for toll booth facilities and operators and allowing tolls to be collected while vehicles are operating at highway speeds. Electronic toll collec- tion has been implemented in many cities across the United States and worldwide as a convenient means of collecting tolls without disrupting the flow of traffic. The main objective of electronic toll collection is to allow roadway user fees to be collected instantaneously from the road user at high speeds. Electronic toll collection allows more than double the vehicle throughput of manually operated toll booths and reduces vehicle delay at toll plazas. The increased per lane capacity of electronic toll collection requires fewer lanes. The operating expenses incurred from electronic toll collection equipment are typically offset by the reduction in staff required to operate the facility.

122 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Shifts Demand to Other Time Periods Electronic toll collection provides a means for counting the number of vehicles travel- ing along a given segment of roadway. This information can be used in conjunction with ITS to notify the traveling public of congested conditions. On receiving informa- tion that a certain freeway segment is experiencing congestion, potential users may divert to other routes or plan their trip during a different period, thereby reducing demand on the roadway during peak times and reducing the associated nonrecurrent congestion. This effect can be magnified through the use of variable toll pricing schemes, which have been used in many locations. By charging road users a premium to access the roadway during times of peak demand, motorists are encouraged to shift their trip to other facilities or time periods. When variable tolls are used, drivers should be noti- fied of the premium pricing as far in advance as possible so they may have the greatest number of alternative options possible. Although variable tolling is often used during recurring congestion by increasing prices during daily peak hours, it can also be used to reduce demand during nonrecurrent congestion caused by special events, road con- struction, or lane-blocking incidents. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of electronic toll collection systems at reducing nonrecurrent congestion: • Traffic demand • Traffic demand profile (roadway segments with short periods of very high demand surrounded by periods of moderate demand will tend to be especially improved by this treatment) • Existing infrastructure for relaying information to the traveling public (ITS, elec- tronic message boards, radio) • Availability of alternative routes with surplus capacity Cost Initial costs for field equipment used in electronic toll collection can be significant. Compared with the costs to build and operate full-scale toll booth plazas, however, electronic toll collection represents a significant reduction in cost. Employing field equipment on existing structures for the purpose of managing nonrecurrent congestion would further reduce the overall cost of implementation. If electronic tolling already exists, the cost of using the system to reduce non- recurrent congestion will include any necessary ITS to relay congestion information to travelers, as well as any expense associated with varying the toll price with non- recurrent congestion.

123 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Overheight Vehicle Detection and Warning Systems Description and Objective This treatment is designed to detect overheight vehicles and activate warning devices to alert those vehicles of oncoming overhead height restrictions. An overheight vehicle is any vehicle containing a load whose height (legal, permitted, or illegal) exceeds the height of an approaching overhead facility. The two primary types of vehicle detection systems are optical systems and laser radar. These systems function similarly, with a transmitter and receiver creating a beam at a desired height. When a tall vehicle passes, the beam is broken, causing the detector to activate a beacon or signal. The primary objective of an overheight vehicle detection system is to reduce the number of overheight vehicle collisions with overhead facilities such as bridges or tunnels. These overhead facilities may be permanent (e.g., a tunnel) or temporary (e.g., construction areas). A secondary objective is to provide appropriate advance notifica- tion so drivers can divert to another route without causing traffic delays. How Treatment Reduces Nonrecurrent Congestion Reduces Number of Overheight Vehicle–Related Crashes Overheight vehicle detection systems are deployed to reduce the annual number of overheight vehicle–related incidents and crashes. By notifying drivers of potential clearance problems in advance of an overhead facility, this treatment can reduce the total number of such crashes. With fewer crashes blocking lanes and negatively affect- ing traffic flow, nonrecurrent congestion is reduced, and reliability is improved. Reduces Number of Overheight Vehicle–Related Lane-Blocking and Shoulder-Blocking Events In some cases, drivers of overheight vehicles may realize that their vehicle is too tall to clear an upcoming obstruction, but this realization comes too late to divert to another route. The driver will stop the vehicle before colliding with the obstruction, but blocks the lane or shoulder, thus impeding other traffic. When overheight vehicle detections systems are employed, such that drivers of overheight vehicles are made aware of upcoming obstructions with enough notice to divert to another route, the number of these lane-blocking and shoulder-blocking events are reduced. This reduction in events results in reduced nonrecurrent congestion and improved reliability. Factors Influencing Treatment Effectiveness The following factors are important in determining the effectiveness of overheight vehicle detection systems at reducing nonrecurrent congestion: • Annual number of overheight vehicle crashes • Annual number of overheight vehicle lane-blocking events • System effectiveness at notifying drivers in time to avoid collision • Available alternate routes for overheight vehicles

124 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Cost Cost estimates will vary depending on the type of system deployed. A simple, lower- cost system could be an optical system, consisting of an activated flash beacon mounted above a static sign using solar power and wireless communication. A more complex, higher-cost system would be one that has a series optical system, dynamic message signs with activated flashing beacons, cameras, high bandwidth communication, and commercial power. EMERGENCY RESPONSE NOTIFICATION Reference Location Signs (Emergency Reference Markers) Description and Objective Reference location signs provide precise location information to roadway users, inci- dent management responders, and highway maintenance personnel. The objectives of reference locations signs are as follows: • Aid roadway users in their trip progress assessments • Serve as alignment markers on rural routes, especially at night or in other low- visibility conditions • Facilitate road maintenance activities such as repair operations, clearance of debris, and highway inventories • Assist in accurate reporting of emergency incident and traffic crash locations to reduce response time and inform other motorists of incidents that may affect rout- ing decisions The second and third objectives listed above can reduce nonrecurrent conges- tion by shortening the duration of the incident. Accurate location information allows maintenance crews to quickly remove debris from the roadway that may be blocking the travel way; it also allows emergency responders to arrive at the scene of a crash quickly, so that investigation and vehicle removal can occur quickly. Quicker identifi- cation of incidents typically results in faster response and clearance times and allows highway operations to return to normal sooner than they may otherwise have. The benefit of rapid identification and response is considered here. Reference location signs are typically installed along Interstates and other major highways at mile increments. The marker references the mile post along the roadway, most often numbered from a state line, and correlates with the exit numbers along the route, as illustrated in Figures 4.17 and 4.18. Intermediate reference markers may be used at one-tenth or two-tenths of a mile increments and include both the mile post and a decimal. Various types of highway reference markers are shown in Figure 4.18.

125 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Incident Response Time Reference location signs allow people involved in or reporting an incident to provide detailed information about the exact location of the incident. This action results in faster response and clearance times, reducing congestion and improving reliability. Figure 4.18. Examples of highway reference markers. Source: MUTCD (FHWA 2009). Figure 4.17. Highway reference marker on I-470.

126 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Reduces Mainline Demand During an Incident Reference location signs promote the communication of accurate incident location information that can be relayed to motorists through advanced traveler information systems such as dynamic message signs or local radio stations. When motorists are made aware of congested conditions along their route, they are able to divert to other routes, thereby reducing the mainline demand. This reduced demand helps improve operations while the incident is being cleared, reducing delay and improving reliability. Reduces Frequency of Incidents Frequently spaced intermediate reference location signs (such as every tenth of a mile) may serve as roadway delineators, especially at night or during inclement weather. These signs may help keep drivers on the road when the edgeline may be difficult to see, which reduces the frequency of lane-blocking incidents and crashes. With fewer lane-blocking incidents, freeway congestion is reduced and reliability is improved. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of reference location signs in reducing nonrecurrent congestion include the following: • Existing emergency response time • Familiarity of average driver with area (emergency reference markers will have a stronger impact on roadways with a high percentage of drivers unfamiliar with the area) • Spacing of emergency reference markers (a closer spacing will increase the prob- ability that a driver will be able to determine his position and accurately report to a 911 operator) • Size of sign • Reflectivity of sign • Presence of a traffic management system that can identify crash locations before call is made Cost The cost of installing reference location signs will depend significantly on the distance of the roadway and planned spacing. The reference markers will also require periodic inspection and replacement. Roadside Call Boxes Description and Objective Roadside call boxes are telephone stations located in close proximity to a roadway. They are useful in locations with inadequate cell phone reception, such as tunnels. Modern call boxes are capable of informing dispatchers of the location where the call originated, which aids in directing emergency personnel to the scene more quickly.

127 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION The objective of roadside call boxes is to notify emergency personnel of an inci- dent quickly, thus decreasing the response time of emergency personnel. The informa- tion relayed via a roadside call box may also be used by ITS to notify drivers of adverse conditions ahead (e.g., by using variable message boards or radio announcements). Typical Applications Historically, call boxes were placed at regular intervals (usually a mile or less) along major routes by the agencies that use them. But as cell phones have become more prevalent, call boxes may not be needed in as many locations. Emergency call boxes can be most beneficial in locations with poor cellular phone service (such as in tunnels or along mountain passes) and in specific areas where congestion-causing incidents are frequent. Call box phones are typically linked directly to a call center or emergency re- sponse organization. The call center staff member takes information from the motorist and dispatches the appropriate services (emergency assistance, tow truck, medical). Location information can either be obtained from the motorist (a sign on the box typically includes a location identification code) or transmitted from an automated identification system located within the call box. The phones are usually operated by pushing a single button; that is, they do not allow calls to be made to other phone numbers. On some phones, a color button sys- tem allows the caller to identify the specific service they need. Some phones are also equipped to handle TTY calls for the hearing impaired. Roadside call boxes may be powered either by electrical power or solar battery power. Telecommunication is either wireless or provided by wire line. In locations with an ITS traffic management system, information gathered from call box users can be relayed to other motorists through changeable message signs or via local radio stations to indicate the potential for congestion or lane blockage. Such announcements may help reduce demand on the affected highway segment by allowing motorists to choose another route. How Treatment Reduces Nonrecurrent Congestion Reduces Incident Response Time Roadside call boxes can help to notify emergency personnel of incidents relatively quickly and accurately regarding the location of a disabled vehicle. By providing per- sonnel with quicker and more accurate information, this treatment can reduce the emergency response time and allow the incident to be cleared sooner. This reduction in lane-blocking time leads to a reduction in nonrecurrent congestion and improved reliability. Reduces Mainline Demand During an Incident Roadside call boxes can be used to communicate to a traffic management center that an incident has occurred. The traffic management center may then notify drivers be- fore they reach the incident by using ITS technology (e.g., variable message boards). Drivers who receive this information may divert to other routes to avoid the incident and subsequent congestion. By reducing the demand during the incident, congestion is reduced and reliability is improved.

128 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness The following factors may influence the effectiveness of roadside call boxes at reducing nonrecurrent congestion: • Cell phone reception (the less reception available, the more useful this treatment will be) • Efficiency of notification system at relaying information to emergency responders • Traffic demand volumes • Presence of ITS • Capability of informing motorists of lane-blocking incidents through variable message signs or local radio • Frequency of call boxes and uniformity of identifying signs and locations Cost Cost estimates will vary depending on ownership (agency or private) and the type of system (commercial power and communication, commercial power and agency com- munication, solar battery and commercial communication). Factors that may affect the cost to implement call boxes include the following: • Hardware costs for phones, stands, cases, and other elements • Installation of phone cable • Installation of new power source or connection to existing power source • Ongoing phone service fees and electricity costs WEATHER Fog Detection Description and Objective Fog detection systems automatically detect the presence of fog and then post warning messages on dynamic message signs upstream of the fog detector to notify drivers that they are entering a portion of roadway where fog is present. The objective of fog detection systems is to reduce the number of fog-related inci- dents and crashes by notifying drivers of reduced visibility ahead and inducing them to reduce speeds and proceed with caution. How Treatment Reduces Nonrecurrent Congestion Reduces Number of Incidents Fog detection systems are deployed to reduce the annual number of fog-related in- cidents and crashes. By notifying drivers in advance of a roadway section with low visibility, these systems encourage drivers to slow down and proceed with caution, resulting in fewer crashes. With fewer crashes blocking lanes and negatively affecting traffic flow, nonrecurrent congestion is reduced and reliability is improved.

129 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION In Best Practices for Road Weather Management, Goodwin (2003) describes how, from 1973 to 1994, there were 200 fog-related accidents with 130 injuries and 18 fatalities on I-75 in Tennessee. Accidents were reduced after implementation of the 19-mi Tennessee Low-Visibility Warning System (a fog detection system) on the affected stretch of the Interstate. Factors Influencing Treatment Effectiveness Several factors may influence the effectiveness of fog detection systems at reducing fog- related crashes and incidents: • Expected annual number of fog events affecting roadway operations • Effectiveness of sensors at recognizing fog and activating warning signs • Driver behavior (e.g., whether motorists follow instructions to reduce speed) Cost The cost of a fog detection system varies depending on the size of the system, the ex- tent of automation, and the means used to notify drivers (e.g., dynamic message signs may need to be constructed). Roadway Weather Information Systems Description and Objective A roadway weather information system (RWIS) gathers meteorological information and relays it to highway agency and maintenance personnel. RWIS is composed of environmental sensor stations in the field; a communication system for data transfer; and central systems to collect field data from numerous environmental sensor stations, which measure atmospheric, pavement, and/or water level conditions. Atmospheric data include air temperature and humidity, visibility distance, wind speed and direc- tion, precipitation type and rate, tornado or waterspout occurrence, lightning, storm cell location and track, and air quality. Pavement data include pavement temperature, pavement freeze point, pavement condition (e.g., wet, icy, flooded), pavement chemi- cal concentration, and subsurface conditions (e.g., soil temperature). Water level data include tide levels (e.g., hurricane storm surge) and stream, river, and lake levels near roads. Central RWIS hardware and software are used to process observations from environmental sensor stations to develop forecasts and display or disseminate road weather information in a format that can be easily interpreted by a manager. The objective of RWIS is to provide road weather information to highway agency and maintenance personnel to enable them to make more informed decisions. The data collected by RWIS can be used to create computer models of weather patterns and continuously calibrate these models. The model weather predictions can be useful to highway agencies in making a determination to close roadway segments in advance of a major storm. The model forecasts can also help agencies be better prepared to respond to a storm with treatments such as anti-icing or deicing chemicals and snow removal equipment.

130 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Figure 4.19 presents a photo of a RWIS at Turner–Fairbank Highway Research Center. How Treatment Reduces Nonrecurrent Congestion Reduces Frequency of Incidents RWIS information can be used by maintenance crews to prepare for deicing, anti- icing, and snow removal treatments in advance of a winter storm. By helping to im- prove winter maintenance operations and pavement conditions, RWIS technology may reduce winter-related crashes. By eliminating crashes, this treatment reduces the lane-blocking time associated with these crashes, which results in a decrease in non- recurrent congestion and improved reliability for the roadway. Figure 4.19. Road weather information system at the Turner–Fairbank Highway Research Center. Source: Zaccagnino (1996).

131 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of RWIS at reducing nonrecurrent conges- tion include the following: • Sophistication of system o Numbers and types of sensors o Accuracy of prediction models • Effectiveness of communication system between RWIS and maintenance personnel Cost The cost of an RWIS system varies depending on the size of the system, as well as weather-prediction model development and ongoing calibration. If certain infrastruc- ture is already in place (e.g., ITS for managing freeway lanes and dispatching emer- gency personnel), the cost of a RWIS may be reduced. Flood Warning Systems Description and Objective Flood warning systems automatically warn motorists of potentially hazardous flood conditions on the roadway ahead. There are two types of flood warning systems: ac- tive and passive. Passive systems consist of warning signs indicating a location on the roadway that may flood or be susceptible to standing water during heavy rains. Active warning systems consist of a sensor to detect high water levels or water on the roadway and a variable message sign (or flashing lights on a static sign) to warn motorists. Some flood warning systems include the capability of closing the roadway with physical barriers, which consist of either automated railroad crossing–type gates or manually placed barricades. The following are components of flood warning systems (Boselly 2001): • Sensors to detect the level of water at various locations, such as streambeds, bridges, and road surfaces • Data processing technology to store the data of all locations • Components that provide an automated alert to emergency response personnel • A monitoring subsystem such as a website database that receives data from the field sensors and provides readily accessible and usable information to emergency managers and motorists • Automated or manually activated signage • Automated or manually activated traffic control components, such as gates to physically barricade the roadway The objective of flood warning systems is to notify motorists that the roadway ahead may be underwater from nearby streams and rivers and to discourage or pro- hibit motorists from entering that section of the roadway.

132 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION How Treatment Reduces Nonrecurrent Congestion Reduces Number of Incidents Flood warning systems are designed to reduce the number of incidents related to flooded roadways and to prevent vehicles from driving into a situation in which they are “trapped” by a flooded roadway. Like other weather emergency treatments, such as gates that close freeway ramps during snowstorms and contraflow lanes during hurricane evacuations, this treatment is not designed to reduce trip delay or increase reliability, but rather to minimize incidents and improve safety. Factors Influencing Treatment Effectiveness The following factors may influence the effectiveness of flood warning systems at re- ducing flood-related crashes and incidents: • Expected annual number of flood events affecting the roadway • Effectiveness of sensors at recognizing flood conditions and relaying a message to personnel or automated gates • Response time of personnel to manually set barricades (if applicable) Cost The cost of a flood warning system varies depending on the size of the system, the ex- tent of automation, and the means used to notify drivers (e.g., dynamic message signs may need to be constructed). Wind Warning Systems Description and Objective Wind warning systems warn motorists of potentially hazardous wind conditions along the roadway ahead. High winds can cause vehicles to swerve, leave the roadway, col- lide with other vehicles, or overturn; trucks are particularly susceptible to these prob- lems. Since high winds are often less visible than other inclement weather conditions (e.g., snow or fog), wind warning systems can be very valuable for apprising drivers of dangerous conditions and urging them to drive more slowly and carefully. The simplest wind warning system would be the installation of static signs at high- way entrance ramps in areas prone to high winds. Many static signs are supplemented with wind socks that provide drivers with real-time visual information about current wind conditions. Some static signs are accompanied by flashers that are activated dur- ing high winds. More advanced wind warning systems include variable message signs, which automatically show wind advisories based on wind speed sensor data. Some policies prohibit certain vehicles from entering a highway or bridge during high wind conditions, and variable message signs can post these prohibitions. The objective of wind warning systems is to provide information about potentially hazardous driving conditions to drivers, especially those in vehicles especially suscep- tible to wind such as trucks, RVs, or vehicles with oversized loads. These warnings

133 DESIGN GUIDE FOR ADDRESSING NONRECURRENT CONGESTION discourage drivers from traveling during the windy conditions, or encourage them to find an alternative path or proceed with greater caution. How Treatment Reduces Nonrecurrent Congestion Reduces Number of Incidents Wind warning systems are designed to reduce the number of crashes and incidents caused by high winds. Like other weather emergency treatments, such as avalanche warning systems and gates that close freeway ramps during snowstorms, this treat- ment is not designed to reduce trip delay or increase reliability, but rather to minimize incidents and improve safety. Factors Influencing Treatment Effectiveness Factors that may influence the effectiveness of wind warning systems at reducing wind- related crashes and incidents include the following: • Frequency of high wind events that potentially cause crashes or incidents • Effectiveness of sensors at recognizing high winds and relaying message to motorists Cost The cost of installing a wind warning system varies depending on the type of detection used, the type and number of signs used, and the extent to which the system is auto- mated. In general, construction of a wind warning system may cost several hundred thousand dollars.