Data on traffic volume trends on U.S. highways are essential to many transportation functions, including infrastructure and maintenance planning. Traffic trends also are vital to other stakeholders, such as the tire industry, which uses the monthly estimates to determine production schedules.

Each month, the Federal Highway Administration's (FHWA) Office of Highway Policy Information compiles information on trends in traffic volume across the country. "These monthly estimates have grown progressively more relevant to discussions ranging from economic vitality to the future of the Highway Trust Fund," says Doug Hecox, deputy associate administrator for FHWA's Office of Public Affairs. "These data, which have been compiled by FHWA for years, are now among the most highly anticipated data produced by the Federal Government."

FHWA publishes the Traffic Volume Trends reports, which estimate vehicle miles traveled (VMT) on U.S. public roads, within 6-8 weeks of the close of a given month. These data are available to the public at www.fhwa.dot.gov/ohim/tvtw/tvtpage.cfm.

The reports contain data demonstrating the changes in VMT by region and State, VMT changes on rural and urban arterial roads by region and State, and VMT changes on all estimated roads by region and State. Also included are rural and urban volume trends by quarter and graphs of travel trends by month and year. The Web site makes it possible to compare trends over 35 years with monthly VMT estimates dating back to 1970 and annualized data back to the 1940s.

States Gather and Report Data

States are responsible for reporting monthly traffic counts to FHWA through the Travel Monitoring Analysis System, an online portal through which States submit their data. State highway agencies collect the data through permanent, automatic traffic recorders, such as inductive loops, installed on public roads. Approximately 4,000-5,000 continuous traffic counting locations exist nationwide, each of which reports data in 5-10 minute intervals.

FHWA estimates VMT by State and several functional classes of roads. The estimates are based on two sources of data: the data reported by States and the Highway Performance Monitoring System, which pulls data from sensors mounted in roads. Monthly average daily traffic is computed using the States' traffic counts. FHWA analysts use the data to estimate the percent change in traffic for the current month compared with the same month in the previous year. Next, they average the change rates by functional class of road and estimate monthly VMT by combining the change rates for each month with the most recent annual VMT from the Highway Performance Monitoring System.

FHWA runs quality control checks on all data received. Analysts review data reported from the traffic recorders, looking for inconsistencies. Data with inconsistencies caused by any number of factors (such as malfunctioning recording devices, traffic incidents that block roads, and snow) are replaced with estimates from nearby traffic recorders. FHWA uses only data that pass the quality control checks in the Traffic Volume Trends reports. Another criterion: The data collected must represent a minimum of 30 States and 70 percent of the VMT in order for FHWA to publish a report.

Recent Trend: A Decline in Driving

January 2009 marked the 14th consecutive month of declining driving. From December 2007-January 2009, the decline topped 122 billion VMT compared to the same period a year earlier (December 2006-January 2008). In January 2009 alone, travel on all roads and streets was 3.1 percent less, or 7 billion fewer VMT, than January 2008. According to the reports, cumulative travel for 2008 fell 3.6 percent, or by 107.9 billion VMT, compared to 2007.

"We are watching this trend closely," says Hecox. "When it was first identified in late 2007, we believed it had to do with rising fuel prices. However, when fuel prices began to normalize after cresting in July 2008, traffic levels continued to fall--suggesting that the factors may be an array of economic variables. Joblessness may play a role in declining driving, but so too might overall consumer confidence. This change in driving behavior will surely be studied by academics and theoreticians in the years to come but, for now, no specific cause has yet been identified."

The relevance of these data has never been more apparent, Hecox says, especially due to the connection some are making between the trend and the Nation's economy.

For more information, visit www.fhwa.dot.gov/ohim/tvtw/tvtpage.cfm.

Alicia Sindlinger is a contributing editor for Public Roads.