Download Report:

Databook:

Figures:

Chapter 3.0: Austin-Area Mobility Analysis

Congestion on urban roadways is on the rise causing commuters to burn excess gas idling on freeways, preventing the timely delivery of commercial products via freight transportation, and keeping millions from arriving at work and various other engagements as scheduled. The negative economic effects of congestion are growing for all cities. These effects cannot be ignored - congestion is affecting more roadways for longer periods during the day in almost all major urban areas. The Austin area is feeling these effects worse than all other cities of its size.

The 2005 Urban Mobility Report examined and analyzed mobility data for 85 cities throughout the United States. Cities were grouped in categories based on their populations with Austin falling into the medium-size category with 29 other cities with populations between 500,000 and 1 million. The following analysis will, among other things, compare mobility trends in the Austin metropolitan area with trends seen in the medium-size group as a whole. The cities that comprise the Urban Mobility Report's group of medium-size cities are listed below.

Medium (Population Greater than 500,000, Less than 1 Million)

Austin, TX

Honolulu, HI

Toledo, OH-MI

Charlotte, NC-SC

New Haven, CT

Tulsa, OK

Louisville, KY-IN

Sarasota/Bradenton, FL

Akron, OH

Nashville/Davidson, TN

Grand Rapids, MI

Dayton, OH

Tucson, AZ

El Paso, TX-NM

Rochester, NY

Jacksonville, FL

Allentown/Bethlehem, PA-NJ

Springfield, MA-CT

Oxnard/Ventura, CA

Richmond, VA

Albuquerque, NM

Memphis, TN-MS-AR

Hartford, CT

Raleigh/Durham, NC

Bridgeport/Stamford, CT-NY

Fresno, CA

Birmingham, AL

Salt Lake City, UT

Albany/Schenectady, NY

Omaha, NE-IA

For further evaluative purposes this analysis will present peer city comparisons between Austin and the regions of Portland, Tucson, Salt Lake City, Denver, Minneapolis/St. Paul, Atlanta, and Raleigh/Durham. These cities were chosen due to their appropriate sizes, growing metropolitan populations, employment composition, and universities. Additionally, five peer cities are the capitals of their respective states.

Congestion is not a measure that can be expressed as a single, absolute number. Rather, congestion is expressed in terms of delays and travel time, which tend to relate very closely to each other. The travel time index is an area-wide measure of the extra amount of travel time required for trips taken during peak period travel times of the day, generally rush hours.

Delays measure the actual excess travel time during periods of congestion. If a ten-minute trip in normal traffic takes fifteen minutes during peak periods (rush hours) then that means congestion has resulted in a five-minute delay for that trip. The travel time for that trip would be 15 minutes and the travel time index would be 1.5 (1 + Delay/Normal Travel Time). It should be noted that the routes traveled are not taken into account when calculating travel time index and delays. Delays and travel time often tell the same story, but slight differences between them can be indicative of subtle changes in an area's overall congestion picture. For example, rapid growth in the travel time index with lower level increases in delay can be indicative of commuters adjusting travel patterns, by making shorter trips or using public transit, in response to perceived traffic conditions.

This analysis will refer to system-wide congestion, as opposed to segment-based congestion. Traffic analyses that identify congestion on specific roadways refer to segment congestion (e.g., "Traffic congestion on MoPac between Riverside and 35th"). This is the conventional use of the word congestion that is understood by the public as much as transportation engineers.

The analysis conducted as a part of the Urban Mobility Study pertains to system-wide congestion. System-wide analysis approaches congestion from a holistic perspective, taking into account all roadways within the Austin area, with the premise that segment congestion has a spillover effect that increases travel time for all travelers (whether or not they travel on the specific segments).

To illustrate, motorists encountering traffic on a crowded interstate may choose to take an alternate route as opposed to using the congested interstate segment. The act of one motorist leaving the roadway reduces traffic for that segment. If enough travelers leave the roadway, traffic will reduce further, thus reducing travel times and eventually reducing congestion for that segment. However, the travelers leaving the roadway have to go somewhere. If one only looks at changes in the travel time indices of a few segments, the risk of missing spillover effects from congestion on those segments onto arterials or other freeways increases.

Sources

  1. Texas Transportation Institute. 2005 Urban Mobility Report, Texas A&M University, http://mobility.tamu.edu/, May 2005.

List of Figures