Does Travel Time Reliability Matter? - Primer

How Do We Create a Reliable Transportation System?

Determine Reliability

One of the first steps to understanding the current reliability of a transportation system is to implement a travel time monitoring program, which comprises actions a transportation agency can take to track reliability performance and understand the factors that influence travel time reliability. Reliability monitoring programs also help agencies evaluate progress in making a transportation system more reliable, comparing network reliability of one region with another, and justifying investments that enhance reliability.

Monitoring travel time is the basis for providing credible reliability information to system operators and users. Potential users of travel time reliability monitoring programs include system administrators and their staff, highway system operators, transit system operators, freight service providers, highway system users, transit system users, freight system users, and employers.⁽⁴¹⁾ Such information is useful for making better decisions concerning actions they can take to better plan and manage their travel.

A travel time reliability monitoring program has to be able to perform four critical functions. First, it has to measure travel time by fusing information from a variety of sources using statistical analysis. Second, it should be able to evaluate the reliability performance of a given network or link. Third, if a network or link is found to be unreliable, the program should be able to identify the cause of the unreliability and the affected links. This is critical for identifying an effective strategy to correct the unreliability. Finally, it needs to help system operators clearly quantify and visualize the effect of the detected unreliability. These four critical functions will help agencies identify the appropriate actions to be taken to restore system operations or mitigate the effect of the unreliability.⁽⁴¹⁾

Reliability monitoring systems (in combination with other transportation planning and management strategies) can improve the travel time reliability of a region. Given that system managers have a variety of traffic management strategies at their disposal, they can implement appropriate actions to improve reliability or mitigate the effects of unreliability (e.g., deploy more patrol vehicles). Travel time monitoring systems add a valuable decision-support tool to system managers for practical and efficient traffic management.

Table 3 shows reliability improvements from strategies and treatments that reduce delays (shown in order of most to least effectiveness). The treatments that address occasional delays, such as service patrols and road weather information systems, will specifically improve reliability. Any treatment, however, that reduces delays will benefit reliability as well.

Table 3. Evaluation of strategies and treatments that improve travel time reliability.

Reliability Improvement	Strategies and Treatments
Delay reduction of up to 50 percent	National traffic and road closure information Service patrols On-scene incident management (e.g., incident responder relationship, high-visibility garments, clear buffer zones, and incident screens) Work zone management Transportation management centers Traffic adaptive signal controls and advanced signal systems Electronic toll collection
Delay reduction of up to 20 percent	Remote verification (e.g., through closed-circuit television cameras) Pretrip information by 511, websites, subscription alerts, and radio Road weather information systems Dynamic message signs Geometric design treatments Signal retiming and optimization Advanced transportation automation systems, signal priority, and automated vehicle location Ramp metering and ramp closure Congestion pricing (e.g., area wide) Managed lanes (e.g., high-occupancy vehicle lanes, high-occupancy toll lanes, and truck-only toll lanes)
Delay reduction of up to 10 percent	Planned special events management Freight shipper congestion information and commercial vehicle operations Driver assistance systems Traffic-signal preemption at grade crossings
Other improvements	Driver qualification (e.g., driver training) Automated enforcement Probe vehicles as a means to provide near real-time travel time estimation Geometric design improvements Variable speed limits
Unknown benefits to date	Access management (e.g., driveway location, raised medians, channelization, and frontage roads) Changeable lane assignments (e.g., reversible and variable) Integrated multi-modal corridors Travel alternatives (e.g., ride-share programs, telecommuting, home office, and video conferencing)

Improve Reliability

There are several actions that can be taken to improve travel time reliability. These actions can be grouped based on the similarity of their technical nature or issues addressed. These groups of actions include better management of incidents and other traffic disruptions, capacity additions at bottlenecks, and use of better business and organizational processes to improve reliability. The following sections explore each of these in more detail.

Improve Planning and Response to Incidents, Roadwork, and Special Events

Work zones, special events, crashes, and weather are major sources of unreliability. Incident-related congestion and delays can be reduced with incident management strategies that increase incident detection rates and reduce incident clearance time to restore traffic back to normal conditions. Delays due to special events are caused by sudden traffic surges and can be reduced by proactive management and control of traffic, such as traffic rerouting, temporary capacity addition in the direction of major traffic, and control of traffic at road entry and exit points. Advanced work zone management reduces the effects of work zones by managing the scheduling and duration of work zones and adequately controlling traffic near the work zones. Similarly, a road weather program can play a significant role in mitigating the safety and mobility effects of adverse weather on traffic operations and providing accurate and relevant weather information to travelers. Detailed discussions of advanced incident, work zone management, special event, and weather strategies are presented in the following sections.

Traffic Incident Management Services

Traffic incident management services refer to rapid and efficient removal of debris, roadkill, stalled vehicles, and vehicles involved in crashes, as well as detection of incidents. It also refers to coordination of agencies involved in clearing the incident, such as the fire department, police department, paramedic services, and towing companies. The benefit of rapid and coordinated incident management is that unexpected incident-induced delays are minimized, and injured people are treated more quickly. Rapid incident response also reduces the occurrence of secondary incidents resulting from traffic disturbances due to primary incidents.

Along the busy shipping corridor of I-710 leading from the ports of Long Beach, CA, and Los Angeles, CA, rapid incident management services have helped improve travel time reliability.⁽⁴⁴⁾ Considering a 30-minute free-flow trip, the typical average delay on this route was reduced from 8 to 5 minutes due to rapid and coordinated incident management. Similarly, the travel time on the worst days was reduced by 9 minutes.

A time-consuming component of traffic incident management is tediously measuring tire marks as well as locations of debris and pavement marks at crash scenes. Technologies are available to expedite this process. Photogrammetry is one way to get precise measurements from photographs. For example, Indiana uses photogrammetry-based crash reconstruction. It allows police and crash reconstruction personnel to map a crash scene faster than traditional methods, which rely on field measurements that must be taken while lanes are blocked. Indiana State Police reported that traditional crash reconstruction takes on average 2 hours and 46 minutes, while photogrammetry-based crash reconstruction takes only 59 minutes on average. This use of photogrammetry translates to an average of 1 hour and 47 minutes saved per road closure scene.⁽⁴⁴⁾

Work Zone Management

Work zone management strategies that reduce work zone impacts and generate time and cost savings are crucial for improving travel time reliability. About 24 percent of non-recurring traffic delays are attributed to work zones. Effective work zone management can reduce work zone-related delays by 50 to 55 percent, which can create an opportunity to improve travel time reliability.⁽¹²⁾

Techniques that reduce the impact of work zones include monitoring traffic and making adjustments to traffic control; providing traveler information about work zone locations, delays, and possible alternate routes before and during trips; and effectively planning and coordinating work zones. Examples of these techniques include the following:

• Management of work zone traffic: Traffic conditions at and upstream of work zone locations are better managed if drivers are given information so that they have a better perception of the work zone and how traffic is managed around it. This can be achieved by utilizing queue warning and speed management techniques to enhance safety and reduce traffic delays at work zones.
• Planning and coordination of work zone activities: Sharing information on short- and long-term work zone activities with multiple local and regional agencies creates an opportunity to engage stakeholders, develop an optimal schedule and sequence for work zones, combine multiple projects in a corridor, and coordinate construction work among agencies.⁽⁸⁷⁾ These techniques expedite work zone completion and minimize lane closures and total delay to road users. They also help construction crews with getting materials to the work site.

Special Event Management

Special events (e.g., street closures for parades and high travel demand at stadiums during sporting matches) are a source of delays and unreliability as they suddenly increase traffic demand and diminish roadway capacity. Delays due to special events affect not only individuals traveling to attend the event but also those who are using the adjoining roadways in the vicinity of the event.

In Los Angeles County, it is estimated that approximately 1,000 planned special events happen each year and are attended by millions of people.⁽⁸⁸⁾ When not managed adequately, such high numbers of special events can be a significant source of delay in both urban and rural areas.

Because planned events are scheduled ahead of time, public agencies can obtain information on the location, time, duration, and expected demand from the events. As a result, there is an opportunity to successfully manage the traffic before and during the events to minimize delays and unreliability. Specifically, the effect of such events on delays can be reduced by proactively managing and controlling traffic through tactics such as rerouting traffic, adding temporary lanes in the direction of major traffic, controlling traffic entry and exit points, and sharing real-time information with drivers.

Road Weather Information Systems

Travel time reliability is affected by the existence and intensity of adverse weather conditions, such as precipitation, snow, wind, fog, and sun glare. In addition to disruption of traffic flow, adverse weather can cause damage to infrastructure assets, increase weather-related crashes, and interrupt emergency services. The most severe weather events (e.g., heavy rainfall that causes flooding, severe wind that topples trees, and snow ice) have the possibility of causing complete road closures.

To cope with the effects of adverse weather on reliability, advanced weather detection and response strategies can be deployed. Road weather information systems develop and implement techniques to maintain safety, mobility, and reliability during adverse weather conditions. This is achieved by disseminating information to road users and integrating weather with traffic operations and maintenance procedures. Adequate road-clearing efforts before, during, and after adverse weather events can restore roads to full capacity or near capacity. The statewide reduction in user costs due to a weather responsive traveler information system in Michigan was estimated to be 25 to 67 percent.⁽⁸⁹⁾

Actively Manage Transportation and Demand

Active Transportation and Demand Management (ATDM) is the dynamic and real-time management of transportation demand and supply by influencing traveler behavior. ATDM has three components: (1) active traffic management, (2) active demand management, and (3) active parking management.

Active Traffic Management

Active traffic management is the dynamic management of traffic based on combinations of historic, real-time, and predicted traffic conditions. It refers to improvements in transportation system operations by applying traffic management strategies to maintain and improve highway level of service during peak and off-peak hours. As opposed to only responsive traffic management strategies (reacting to traffic conditions), active traffic management is both responsive and proactive (planning according to anticipated traffic conditions). It can include advanced traffic management strategies and traveler information systems that dynamically manage recurrent and non-recurrent congestion. Some of the most common active traffic management strategies are as follows:

• Adaptive traffic lights: Adjusts the timing of traffic lights to accommodate the real-time traffic in the intersection.
• Variable speed limits: Adjusts speed limits for a segment based on real-time traffic and weather conditions.
• Adaptive ramp metering: Dynamically controls the rate of vehicles entering the freeway through the use of traffic signals on freeway on-ramps that are based on real-time conditions of the main line traffic.
• Queue warning: Provides information concerning the presence of downstream stop-and-go conditions via dynamic message signs to prepare drivers for emergency braking.
• Dynamic en-route information systems: Provides travelers with mobility- and safety-related information (e.g., route guidance and other advisory functions).

Integrated corridor management can be viewed as an application of these strategies. It is a joint management of transportation operations within and between corridors that offers an opportunity to operate and improve traffic conditions in the entire system. In addition, emerging connected and automated vehicle technologies offer a new opportunity for improving traffic management and operations.

Active Demand Management

Active demand management refers to strategies that influence individual travel behavior by providing options to reduce the number of vehicles on a regional transportation network during peak hours. It attempts to reduce congestion and improve travel time reliability by managing the demand side of the transportation equation. The goal is to reduce the strain on traffic operations by maximizing the people-moving capability of the transportation system while reducing the number of vehicles on the network. Some examples of demand management are encouraging road users to shift from using single occupancy vehicles to shared modes of transportation (carpooling, car sharing, and public transit systems), influencing the time of travel and the need to travel, and giving incentives to travel during off- peak hours and discourage travel during peak hours (congestion pricing). Active demand management leads to improved air quality, reduced energy consumption, efficient road capacity usage, and greater traffic mobility. Some of the strategies involving active demand management include the following:⁽⁹⁰⁾

• Dynamic ridesharing: Arranges shared rides with short notice for on-time pick up of travelers using advanced communication technology (e.g., smart phones and social networks).
• Managed lanes: Dynamically changes the qualification of vehicles to use a particular lane (e.g., restricting single occupancy vehicles from using high-occupancy vehicle lanes).
• Dynamic pricing: Charges users in certain lanes or roads based on real-time level of congestion.
• Dynamic routing: Directs road users to use alternative roads that are less congested.
• Dynamic transit capacity assignment: Reorganizes the scheduling and assignment of public transit operations in response to the real-time and predicted travel demand.
• On-demand transit: Provides flexible routes and schedules so that users can customize their transit trips as an incentive to avoid using personal modes of travel.
• Predictive traveler information: Provides pretrip and en-route information to road users based on real-time and historical traffic data.
• Other: Includes strategies that employers and service providers implement to distribute peak-hour traffic (e.g., employer programs for telework and variable work hours and commuter incentives by transit providers).

Active Parking Management

Active parking management refers to optimizing the operations of parking facilities through dynamic management and influencing the behavior of travelers. Since limited parking spaces can be the source of unreliable transportation systems, particularly in city centers with curb parking spaces, active parking management can potentially improve trip reliability.⁽⁹¹⁾, ⁽⁹²⁾ Active parking management uses strategies like dynamic parking pricing, dynamic parking reservation, dynamic way-finding, and dynamic parking capacity. Circling for parking causes congestion, so helping drivers find open parking spaces and effectively pricing parking so that spaces open up more often at busy times can reduce congestion and help drivers arrive at their destination in a more timely manner. Effectively pricing parking near offices and shops may also help balance demand between different travel modes.

Increase Capacity

Uncongested facilities have better reliability, even during minor traffic disruptions, as the extra traffic can be easily absorbed by the available capacity. However, with increases in congestion levels, facilities that operate at capacity or near capacity become quickly unreliable and are prone to excessive delays due to slight disruptions in traffic flow. Therefore, in some cases, adding capacity is the best approach to improving both congestion and reliability.

Capacity addition refers to improvements in the road capacity so that more traffic is accommodated by the facility. Capacity addition is a treatment that is focused on a corridor level and can range from simple repainting of pavement markings to more extensive lane additions. Facilities where capacity has been added exhibit less sensitivity to reliability problems.

Capacity can be added at strategic locations where traffic flow breakdown is frequently observed and congestion is severe. Bottleneck locations are good candidates for capacity additions, as they often result in significant improvement in travel time reliability. Capacity addition strategies include link additions, lane additions, roadway widening, access management, pavement resurfacing, lane delineation treatments, and improved geometric alignment.⁽¹²⁾

Considerations for capacity additions, especially major construction projects, include the cost and timeline, impacts to traffic and local communities, and the possibility that increased capacity may lead to induced congestion (e.g., some drivers shifting from other roads, other times of day, or other modes to the newly expanded road).

In some cases, TSMO can be used as an alternative to adding capacity by instead improving mobility and reliability of existing facilities, which can be implemented in a relatively short period of time and at a fraction of the cost than for capacity addition.

This creates an opportunity for agencies to invest in other areas. In cases where capacity addition is needed, TSMO can be used as a complementary strategy to increase the effectiveness of a capacity addition for a longer-lasting solution to mobility and reliability problems and superior performance.

Change Organizational, Institutional, and Business Processes

There are many techniques that can be implemented additionally or as a complement to the three previous approaches. For example, improving travel time reliability may require an integrated business process. Integrating business processes to improve reliability involves defining specific reliability goals, understanding the existing business processes, identifying activities that need to be changed, monitoring outcomes against reliability goals, and implementing and institutionalizing the process. It can also foster coordination among public agencies and stakeholders that are seeking to improve travel time reliability. Reengineering business processes to improve reliability requires direct intervention with various traffic planning and management strategies, such as traffic incident management, work zone management, planned special event management, road weather management, and traffic control system management.

In the context of integrating business processes for travel time reliability, the following obstacles should be addressed:⁽⁹³⁾

• State and local transportation departments are historically focused on construction and maintenance and are less operations focused and performance-driven.
• Given that travel time reliability was recently introduced as a performance metric, it may not get much attention in the business processes of transportation agencies.
• The stakeholders that contribute to or are affected by reliability business processes usually do not have the same motivations, expectations, and approaches to implementing changes.

Other examples of techniques include the organizational and institutional approaches to enhancing reliability. Many of the strategies that enhance travel time reliability are focused on improvement in highway operations and traffic management. However, this can be successful only if the strategies are complemented by efficient collaboration and coordination among the divisions within an agency and other key public agencies. Effective traffic management at work zones, incidents, and adverse weather conditions requires effective collaboration among multiple organizations that perform the various functions of the system (e.g., traffic management center operators, transit service providers, police departments, fire divisions, emergency medical services, construction firms, maintenance crews, debris clearance crews, safety service patrols, weather information services, and towing companies). Therefore, improvement in travel time reliability should consider a systemic approach to successfully execute programs and activities. FHWA offers capability maturity frameworks on these and other topics to aid agencies in assessing their situation and improving their performance.⁽⁹⁴⁾

Institutional approaches should be considered in improving highway operations and travel time reliability. These types of approaches mainly address non-technical features of an agency.⁽⁹⁵⁾ Non-technical features include the existing culture, leadership, priorities, staffing, resources, relationships, and interactions of an agency. For example, effective work zone management requires an "interrelated sequence of planning, systems engineering, resource allocation, procurement, project development and implementation, procedural coordination, and so forth."⁽⁹⁵⁾ Successful execution of all these components depends on having a supportive institutional setting. The institutional architecture of an agency consists of more than its organization, including leadership, legal authorization, organized responsibilities, staff capabilities, available resources, and working partnerships.

A supportive institutional architecture with the following six key elements is needed to improve transportation system management and operations and enhance travel time reliability:⁽⁹⁶⁾

1. Business process: Refers to a formal scoping, planning, programming, and budgeting process to achieve a reliable transportation system. Restructuring business processes to improve reliability requires intervention with various agency divisions. This also includes improving the technical and administrative activities of transportation agencies and institutionalizing the process. Improved reliability business processes start with setting reliability goals and constantly monitoring and evaluating performance outcomes at various agency divisions.
2. Systems and technology: Defines the approach to building a reliable transportation system by developing system architecture, standards, interoperability, standardization, and documentation to improve transportation reliability. Standard specifications of reliability metrics, objectives, and goals focus the efforts of different agencies on a common outcome.
3. Performance measurement: Determines the metrics, data sources, and data analysis techniques to be used for measuring and evaluating reliability. Tracking reliability data can provide useful information to quantify system performance, identify opportunities to improve system operations, and promote quantifiable accomplishments.
4. Culture: Refers to the values, assumptions, and priorities of an agency. Lack of commitment to advance mobility, level of service, customer and stakeholder support, and accountability lead to inefficient traffic operations and management. Undertaking educational programs on travel time reliability and its importance to road users can significantly change the culture and leadership of an agency.
5. Organization and workforce: Seeks to improve the structure of a transportation agency and improve its workforce capability, professional development, and retention. Construction and maintenance projects may have previously been given significant attention, while traffic management and other functions were left fragmented and understaffed with limited capabilities to improve reliability. Well-trained staff with clearly defined responsibility, accountability, and performance incentives with respect to reliability are required.
6. Collaboration: Refers to improving the relationship and communication between stakeholders, partners (e.g., public agencies, local governments, and metropolitan planning organizations), and the private sector. Lack of clearly defined roles and responsibilities for these groups has an adverse effect on addressing congestion and reliability improvements. Consolidated and stable partnerships always benefit reliability. This can be achieved by agreeing on the operational roles and responsibilities of stakeholders, identifying opportunities for joint operational activities, developing procedures for minimal disruptions, and developing public–private partnerships.

Examples of institutional best practices that State transportation departments have implemented include the following:⁽⁹⁰⁾

• Strong and transparent commitment to operational improvements by developing goals, objectives, and strategies in line with the enactment of the Federal transportation legislation, Moving Ahead for Progress in the 21st Century Act.⁽²³⁾
• Public–private partnership for prompt clearance of incidents as demonstrated by both the FDOT Rapid Incident Scene Clearance and GDOT Towing and Recovery Incentive programs.
• Development of unique contractor requirements for effective work zone traffic control as demonstrated by the Oregon Department of Transportation.
• I-95 Corridor Coalition's Operations Academy that provides a 2-week training to middle and upper transportation department managers on transportation management and operations.

The matrix on the following page presents some possible reliability-related problems, solutions, and suggested reports with practical guidance for State transportation departments. Additional resources with useful TSMO insights for addressing reliability-related issues include the following:

• FHWA primer: Improving Business Processes for More Effective Transportation Systems Management and Operations.⁽⁹⁷⁾
• FHWA primer: Creating an Effective Program to Advance Transportation System Management and Operations Primer.⁽⁹⁸⁾
• American Association of State and Highway Transportation Officials' TSMO Web-based publications: http://www.aashtotsmoguidance.org/.⁽⁹⁹⁾

Table 4. Potential reliability problems, solutions, and resources.

Problem	Possible Solution	Report No.
Reliability is not considered a highway performance measure	View reliability as an additional highway performance measure Initiate reliability monitoring programs and turn reliability data into useful information	S2-L02-RR-1 S2-L03-RR-1 S2-L05-RR-3
Reliability benefits are not incorporated in project appraisal	Consider reliability's benefits in the project prioritization process (e.g., consistent travel time, reduced secondary crashes, reduced emissions, and others)	S2-L05-RR-3 S2-L11-RR-1 S2-L35B-RW-1
Reliability is not integrated into the business process of the transportation agency	Define reliability goals and identify activities that need to be changed to achieve the goals Monitor outcomes against reliability goals	S2-L01-RR-1 S2-L01-RR-2
Understanding of operational strategies and treatments to improve reliability is limited	Identify the strengths, weaknesses, threats, and opportunities of key operational strategies and strategies for improving travel time reliability	S2-L11-RR-1 S2-L03-RR-1 S2-L07-RR-1
Organizational and institutional approaches for improving reliability are not considered	Assess the existing culture, leadership, priorities, staffing, resources, relationships, and interactions of the agency to improve reliability Improve coordination between public and private stakeholders	S2-L06-RR-1
Existing real-time travel information initiatives lack reliability information	Develop a process to provide reliability information to travelers	S2-L14-RW-1 S2-L14-RW-2

All reports listed in this table are available online at http://www.trb.org/StrategicHighwayResearchProgram2SHRP2/Pages/Reliability_Projects_302.aspx.

Visit the TSMO website for more information: https://ops.fhwa.dot.gov/tsmo