Does Travel Time Reliability Matter? - Primer
Why is Reliability Important?
Complex and interrelated supply chains combined with travel needs of the general public require a reliable transportation network.
Unreliability has direct negative impacts to:
Businesses rely heavily on transportation systems. If the system is unreliable, it can affect their profitability, labor costs, fuel consumption and emissions, and the wear and tear and depreciation of their vehicles. Let's explore each of these in more detail.
Unreliable transportation systems cause businesses to incur additional costs due to increases in travel time, travel cost, worker schedule flexibility, and service delivery. Transportation-related goods and services account for more than 10 percent of U.S. gross domestic product (i.e., over $1 trillion in value).(44) Therefore, unreliable transportation systems may impede economic development by diminishing productivity and increasing operation costs of the business (e.g., less freight movement, delayed deliveries, increased fuel consumption, and more vehicles needed to complete the same number of deliveries to ensure they are on time or done by the end of the day, which may mean excess capacity in the fleet when travel is smooth).
Annual estimated cost of unreliability on U.S. roads in 2014:(17)
Warehouses and distribution centers often require just-in-time processing and delivery of supplies, but unexpected delays threaten business efficiency, diminish rapid inventory replenishment, cause lost sales, and increase inventory management costs. For example, because of delay in delivery of supplies, businesses are forced to increase inventories by as much as 5 to 15 percent depending on location and industry.(45) In the chemical industry, up to one-third of inventory is in transit at any point.(11) Therefore, a decrease in transportation reliability increases wait time, dead freight, and overall production costs in the industry. In addition, due to unreliable transportation systems, firms are less able to ship and receive goods within specified time frames, which directly impacts customer satisfaction and overall profitability of supplying the market.(46)
Unreliability, especially around warehouses and distribution centers, affects access and on-time delivery, which disturbs supply chain management. A survey-based study examined sources of delays in the supply chain of logistics in the United Kingdom and found that delays due to unreliable travel time were responsible for 23 percent of the total delivery delays.(47) This rate can be as high as 34 percent in certain industries. In addition, the study reported that a quarter of sampled managers of logistic companies identified unexpected delays as the main source of unreliable logistic operations.
The cost of unreliable trip travel times (including the value of lost time, greater vehicle operating costs, increased emissions, etc.) is incurred by road users.
Due to increasingly unreliable travel times and worsening congestion, logistic and distribution companies are forced to restructure their delivery systems to be competitive in the market. For example, logistic companies attempt to reschedule daytime deliveries to avoid delivery during peak periods and unreliable travel patterns. However, this is often constrained by destination accessibility and customer requirements. In the case of interrelated trips, unexpected delays can have cascading effects. A vehicle delayed by unexpected congestion may have to wait for the next available off-loading slot at a distribution center because it missed its original slot. This can result in amplified effects during subsequent delivery schedules.
Improvements in travel time reliability can lead to increased competitiveness and business productivity resulting from more reliable employee arrival times and access to broader labor markets.(48) This, in turn, has indirect and induced economic benefits. Indirect benefits refer to the increased supplier activities due to a stimulated economy from increased business productivity. Induced economic effects refers to activities related to employees' responding to consumer purchases.
Market accessibility and transportation reliability directly or indirectly affect the labor costs of businesses. This effect can be in terms of lateness of employees and their decreased productivity due to stress from being caught in unexpected congestion. Increased labor costs affect the structure and profitability of businesses providing goods and services. In addition to costs associated with late deliveries due to unexpected congestion, shippers lose out on labor time of drivers as they sit idle waiting for congestion to dissipate. For example, when moving goods from one end of I-5 (San Diego, CA) to the other end (Blaine, WA), a commercial vehicle operator would have to add approximately 6 hours of buffer time to ensure on-time delivery with 95 percent confidence.(10) The addition of the buffer time directly increases the labor cost of shipping businesses.
A nationwide survey of 1,000 representative employers evaluated the employers' view of traffic-related delays.(49) The study found that unreliable commute times for employees are a concern for 25 percent of the employers. These employers reported that managers regularly complain about lateness of employees due to unexpected delays. About 33 percent of employers view traffic-related delays as a moderate to major problem in their businesses. When only large employers are considered, unreliable commute times for employees are a concern for 38 percent of employers, and about 52 percent of these large employers view traffic delay as a moderate to major problem in their business.(49)
While it is difficult to unambiguously quantify the effect of workers' start-time delays and arrival reliability on business operations, a survey of 1,200 U.S. companies in the construction industry conducted by the Associated General Contractors of America reported that the companies incurred $23 billion in added expenses and losses of 3.7 million days of worker productivity annually due to traffic congestion.(50) In addition, 93 percent of the responding firms reported that congestion affects their operations, and 64 percent reported an annual loss of productivity of at least one day per worker.(51) A portion of these losses were caused by unreliable employee commute time.
Most likely, the increased labor costs due to unreliable travel times are transferred to the consumers of goods and services. A practical example of this is an increase in taxi fares due to unexpected delays. The fare for taxis and other on-demand ride-sharing services are normally calculated based on in-vehicle time, waiting time, and distance of the route in addition to other fees, such as base fare, booking fees, tolls, and surcharges.(52) Therefore, if a trip takes longer than expected due to non-recurrent congestion, the fare increases to compensate for the time of the driver. The unexpected increase in fares can be frustrating to passengers.
Fuel Consumption and Emissions
U.S. construction companies incurred $23 billion in added expenses and losses of 3.7 million days of worker productivity annually due to recurrent and non-recurrent delays (i.e., unreliability).(50)
Unreliable transportation systems cause vehicles to consume more fuel and produce more emissions. A study that compared vehicle emissions in rush-hour (i.e., recurrent) and work zone (i.e., non-recurrent) congestion with free-flow conditions found that work zones are associated with the highest fuel consumption rates and the highest carbon dioxide, carbon monoxide, and hydrocarbons emissions.(53)
Non-recurrent traffic congestion due to crashes increases vehicle emissions compared to free-flow conditions. A study that quantified the effect of traffic crashes on vehicle emissions and the quality of air near freeway facilities in South Korea found that communities that were located within a 1-km radius of the crash location were affected by traffic-related air pollution exposure.(54)
Non-recurrent congestion (e.g., incidents and work zones) may initially decrease the instantaneous traffic emissions due to reduced average speed; however, per-vehicle emissions are higher over the duration of the incident compared to free-flow conditions.(55) Under congested and low-speed conditions, per-mile fuel consumption and pollutant emissions are higher compared to free-flow conditions.
An unreliable transportation system that often increases travel time due to unexpected delays increases the exposure risk to pollutants. Emissions from motor vehicles contain pollutants and exposure to pollutants adversely affects health. The health costs of total congestion due to exposure to fine particulate matter and related effects was estimated to be $31 billion (in 2007 dollars) across 83 major cities in the United States, part of which is due to unreliable transportation systems.(56)
Wear and Tear and Depreciation of Vehicles
Unreliable travel times, especially when accompanied by stop-and-go congestion, result in more strain on a vehicle's braking system, engine, and tires as well as increase fuel and oil consumption. This unreliability increases vehicle maintenance and repair costs and accelerates the depreciation of vehicles.
A study of 117 incidents between 2004 and 2006 in Nerang, Australia, estimated the increase in operating costs of vehicles caused by incident-induced delays. The study found that incident-induced delays account for five percent of vehicle operating costs.(57) The study mentioned that an incident occurring in the morning peak hours would result in an increase of total operating costs of the vehicles affected by the incident-induced delay by $21,000 (AUD) over the duration of the incident. This may correspond to substantial financial impacts to road users.
To the Traveling Public
The cost in the forms of extra fuel consumption and emissions and additional wear and tear and depreciation of vehicles due to unreliable travel time is certainly also applicable to the traveling public. However, some of the impacts of unreliable travel are unique to the traveling public and warrant special discussion.
Quality of Life and Stress Levels for Drivers
Some studies have linked a secondary effect of travel time reliability to the health and social life of road users. Unpredictable travel times can have adverse effects on worker productivity, such as lateness for work, absenteeism from work, decrease in concentration after an unreliable trip, exhaustion, and poor sleep quality.(5),(6)
In a study that examined a large sample of long-distance commuters in Europe in various travel modes, uncertainty of travel time of commute trips was identified as the main source of travel anxiety.(59) Similarly, rail passengers who estimated that their commute would be unpredictable demonstrated greater levels of perceived stress and higher elevations of the stress hormone cortisol.(60) In other words, unreliable travel time was found to increase stress of travelers but not necessarily impact work performance.
Unreliable commute time increases commuting stress.(58)
A study compared the stress levels of car and train commuters in metropolitan New York City who had similar commutes, origins and destinations, and socioeconomic backgrounds.(61) The study found that significantly higher levels of stress and negative mood were shown by car commuters than by train commuters. Further analysis indicated that the increased stress and negative mood associated with car commuters was attributed to greater driving effort due to unpredictable travel time.
Travel time predictability is an important contributor in explaining commuting-related stress. A study that examined the commute and stress levels of 323 participants in the Netherlands found that a lower degree of commute time predictability leads to higher commuting stress.(58) In addition, the study found that travel time is perceived to be less predictable when commute time is longer. A similar study involving 418 participants found that travel time variability had a stronger effect on the strain and symptom measures of the participants.(6)
Delays for Commuter and Personal Travel
An unreliable transportation system unnecessarily increases the travel time of drivers (both commuters and those traveling for personal use), leading to late arrival times. Unexpected delays represent an opportunity cost of time, which could have been spent at work or for leisure.
Based on the 2012 to 2016 American Community Survey, there were a little over 139 million workers commuting, and their average commute time was 26.1 minutes.(62) Given this high number of U.S. commuters, an unreliable transportation system can have far-reaching effects. An average American commuter spends about 42 hours stuck in traffic congestion per year, costing an average of $960 a year and 19 gallons of wasted fuel.(62) Part of the total congestion cost is due to unreliability.
Reduction in travel time and unreliability during congested hours by decreasing delays is highly valued by commuters and improves satisfaction with their commute.(63) On the other hand, if travel time doubles due to unexpected congestion, commuters are likely to be very dissatisfied about their commute.(63) This highlights the importance of reliable transportation systems to commuters.
To Local Governments
An average American commuter spends about 42 hours stuck in traffic per year, costing an average of $960 and 19 gallons of wasted fuel per year.(62)
The transportation system provides a critical service to the traveling public. Whether they use roadways, transit, or other forms of travel to get to work or special events, they depend on it to get where they need to go. When the system doesn't deliver on a reasonable expectation of travel time, they lose confidence in the Government's ability to perform this basic service.
Litigation and the Public's Confidence in Government's Ability to Perform a Basic Service
Unreliable travel times during work zone activities or special events have been the cause of litigation between residents and public agencies. An example of this is the construction of the Zoo Interchange in Milwaukee, WI, that resulted in congestion that adversely affected the work commutes of residents.(64) This sparked a lawsuit that was filed by Milwaukee civil rights groups against the Wisconsin Department of Transportation (WisDOT) in 2012. The lawsuit was settled in court-sponsored mediation under the terms that WisDOT provide $13.5 million in transit funding to ease the non-recurring congestion in the region during the construction of the interchange.
Confidence of Transit Users on Public Transportation Services
Long wait times and unreliable travel times of public transportation services are the two most important factors that negatively affect transit users' confidence in public transportation and ridership rate.(65) Transit passengers stated that knowledge of when their bus would arrive and whether it reliably arrives on time are the most important factors affecting their decision to ride transit.(66) Therefore, reliable service and journey time are critical for attracting and retaining transit users.
A survey-based study that investigated the perception of public transportation users of the San Francisco Muni transit system found that the confidence of users in the transit system declines because of delays and unreliability in the transit service.(67)
Providing and maintaining reliable transit service is crucial not only for transit user satisfaction, but it is also important for transit operators. Reliable service improves transit operator efficiency, reduces operating costs, and increases revenues by attracting and retaining users.(68)
A transportation system that is unreliable can negatively affect safety and security of both road users and non-road users. An unreliable system increases safety risks, affects emergency response, and slows disaster recovery.
Increases Safety Risks
Travel time unreliability and crash frequency have a cyclical relationship. Crashes cause travel time to be unreliable as the disabled vehicles block part or all of road lanes, and travel time is increased compared to normal crash-free operations. This, in turn, increases the likelihood of a secondary crash, which, in turn, adversely affects travel time reliability.
Unreliable travel conditions can promote risky driver behavior, which can lead to traffic incidents and secondary crashes.
A recent study examined the relationship between crash frequency and travel time reliability on a 20-mile urban expressway located in Orlando, FL.(69) The study found that unreliable travel conditions pose higher crash risks to motorists (i.e., lower travel time reliability would significantly increase crash frequency). In regard to the effect of unreliability on crash type, unreliable travel time tends to cause more multi-vehicle crashes, especially rear-end and sideswipe crashes, than single-vehicle crashes. The reason is that unreliable travel conditions can promote unexpected driving behaviors and risky interactions between vehicles, thus leading to more multi-vehicle crashes. In addition, the study compared the performance of crash frequency estimation models with and without variables that directly estimate travel time reliability. The results indicate that crash frequency models that included direct travel time reliability measures outperformed the traditional analysis that did not include direct travel time reliability measures. The study recommended incorporating direct measures of travel time reliability for better safety analysis of facilities.
Another safety risk of an unreliable transportation system is the exposure to secondary incidents. Secondary incidents are incidents that would not have occurred unless another incident (a primary incident) occurred earlier and in close proximity. Motorists accustomed to free-flowing traffic may not recognize the queue building up due to a primary incident that occurred earlier in time, and they may end up in a secondary rear-end crash. In addition, rubbernecking and distracted driving while looking at a primary incident scene can lead to secondary incidents. Secondary incidents are estimated to represent about 20 percent of all incidents and about 18 percent of freeway fatalities.(70), (71)
Primary incident type, primary incident clearance duration, and time of day are the most significant factors that increase the likelihood of secondary incidents.(53) For each additional minute of primary incident clearance, the likelihood of a secondary crash increases by 2.8 percent, causing a significant safety risk to road users.(72) Clearing secondary incidents can be difficult because first responders are already engaged in clearing the primary incident and are delayed in resolving the secondary incident, which results in additional strain to the incident response staff.
Unreliable transportation systems also increase the safety risk of first responders, mostly because of secondary incidents. Incidents involving emergency responders often occur as a result of the responder being struck by a passing vehicle while working at a traffic incident scene. For example, a rubbernecking motorist looking at a crash scene strikes a paramedic attending a vehicle crash victim. In 2008, 13 police officers were killed from being struck by vehicles, mostly while directing traffic at incident scenes or attending to traffic stops.(73) In 2013, six firefighters were struck and killed by vehicles while responding to roadway crashes and incidents.(74)
Affects Emergency Response
Damage costs increased between $130 to $360 million per year in California due to traffic slowing arrival of first responders for fires and emergencies.(79)
Response time by first responders is one of the most decisive factors that determines mortality rates, healthcare costs, and level of property damage in traffic and other incidents. A survey of first responders registered with the Alabama Department of Public Health found that traffic congestion, on average, delays the response time of first responders by nearly 10 minutes.(75) Any unexpected delay in addition to the recurrent delay exacerbates the late arrival of the first responders.
For situations that are life-threatening, delayed response time for first responders due to unreliable travel time can have a detrimental effect, as rapid interventions in the first few minutes can make the greatest difference. Rapid arrival at traffic crash scenes and rapid conveyance to hospitals is critical for victims with life-threatening medical conditions. In the event of cardiac arrest, survival rates decrease by 7 to 10 percent for every minute of delay.(1)
Healthcare costs increase due to late arrival when seeking treatment. Delayed patient arrival at hospitals is associated with longer stays in the intensive care units, additional health complications, costly procedures, and longer recovery periods.
Similarly, delayed arrival of first responders increases property damage. For example, response times of fire services have a direct correlation with the magnitude of the structural damage of buildings on fire. Building fires are estimated to grow by 20 percent per minute, which results in an average additional damage in the range from $4,000 to $6,000 per minute.(76) Similarly, the monetary increase in damages from fires and emergency medical services due to traffic affecting quick arrival of first responders is estimated to be $130 to $360 million per year in California.(79)
Unreliable transportation systems increase the time it takes first responders to arrive at an incident scene, which poses a significant risk of increased mortality rate, health complications, and property damage.
Slows Disaster Recovery
Travel time reliability is also important to consider as it relates to emergencies and disaster recovery. Traffic demand in areas affected by disaster can be significantly higher due to the evacuation and other activities, which increases the likelihood that the transportation infrastructure will be unreliable. Urban areas are becoming more vulnerable to natural disasters (e.g., floods, rising sea levels, hurricanes, and landslides) due to changes in weather patterns and climate. During these emergencies and disasters, rescue routes need to be reliable enough to maximize the effectiveness and efficiency of evacuations as well as mobilize disaster relief resources and operations.
Effectiveness of recovery operations from disasters and other national emergencies not only requires the road transportation system to be reliable but also resilient. Infrastructure resilience refers to the ability to reduce the magnitude and duration of destructive events by designing the infrastructure to be robust, resourceful, rapidly recoverable, and adaptive.(80)
In summary, a reliable transportation system is critical to all those who use it. So, how do we create a reliable system? First, we must determine the system's current level of reliability and then take action to improve it. We will explore these topics in the next section.
Costs of Unreliability — by the Numbers
Unreliability costs represent costs assumed by road users because the trip travel time is unreliable. In a conservative appraisal of unreliability using 2005 data, the annual cost of unreliability in the United States was estimated to be $10.1 billion (in 2007 value).(81) The total annual unreliability costs in very large and large urban areas were found to be $7.31 and $1.47 billion, respectively, while in medium and small cities, it was $0.14 and $1.17 billion, respectively (in 2007 value). The annual total costs of congestion (including the value of additional travel time, reliability, vehicle operating costs, emissions, and mobility) in 2005 were estimated to be $85.4 billion.
Unreliability-related costs were estimated to be about 12 percent of total congestion costs, though it can be higher in larger urban areas. Assuming the proportion of unreliability to the total congestion costs remain the same, the cost of unreliability in 2014 would have been slightly higher than $19 billion (depending on a total congestion cost of $160 billion as estimated by the 2015 Urban Mobility Scorecard).(17)
United States Department of Transportation - Federal Highway Administration