In the 1950s and 1960s the federal government undertook a massive effort to build the interstate freeway system in the United States. While this system was envisioned primarily as to serve inter-state (hence the name) and inter-city traffic, it has come, over time, to be the backbone of intra-city transportation systems serving individual cities within the country. The overall interstate system was completed by the early 1970s and has not seen significant increases capacity since that time. However, the usage of the system, particularly in urban areas, has increased dramatically since that time. According the Federal Highway Administration (FHWA), during the last decade, metropolitan area traffic on the interstate system has increased by 30 percent, and FHWA projects that this increase will be 50 percent over the next ten years. The Twin Cities are no different from other US cities. In fact, our levels of increase in congestion in recent years are among the highest in the nation.
Due to increased societal concern with environmental, social, and aesthetic impacts since the time of the initial construction of the freeway system, it is much more difficult and expensive to physically increase freeway capacity than it was to initially construct it. Thus, the problem which transportation professionals face is “how do we wring the most use possible out of the infrastructure which is in place?”
One approach is to reduce demand for the over-extended capacity through efforts strategically restricting downtown parking supply, promoting car-pooling, telecommuting, and flex-time work scheduling (allowing non-peak commuting). This approach, referred generally to as Travel Demand Management (TDM), is a very important part of the effort. However, the focus of this case study is the converse perspective on the problem: Transportation Supply Management (TSM). TSM represents the approach of operationally managing the use of existing infrastructure as efficiently as possible so as to maximize the effective capacity of existing roadway systems.
There are a broad range of TSM strategies which are available, including: signal coordination, reversible lanes, HOV lanes, an variable message signs (VMS). One of these strategies is the focus of this case study: ramp meters.
Freeway ramp meters are traffic control signals which are installed at entrance ramps to freeway systems and are used to meter traffic onto the mainline at design rates. There are two primary functions which ramp meters serve:
· to break up “platoons” of vehicles entering the freeway at given access points (thus allowing merging with less disruption to the mainline operations), and
· to hold vehicles temporarily “in storage” on ramps to improve flow characteristics on the mainlines during peak travel times.
Freeway ramp metering has been used as a control technology since the 1960s. The first comprehensive assessment, funded by the Federal Highway Administration (FHWA), was published in 1989. An updated version of this report was published in 1995. According to the 1995 report, the following Metropolitan areas had more than 50 ramp meters:
· Chicago, IL
· Los Angeles, CA
· Minneapolis/St. Paul, MN
· New York, NY
· Orange County, CA
· Phoenix, AZ
· Portland, OR
· San Diego, CA
· San Jose/San Francisco, CA
· Seattle, WA
In addition, 13 other North American Cities used ramp meters (less than 50 meters per city in these cities).
As of May 2000, there were no national standards regarding the installation and operation of ramp meters. Within the basic framework of the goals of ramp metering, transportation authorities can adopt operating policies somewhere within the following extreme cases:
The Twin Cities has the most aggressive ramp metering program in the country. It has over 425 meters throughout the metro area and, at least prior to testing done in 2000, these meters were operated under the first of the two “extreme” operating priorities identified above—they focused upon maximizing flow on the mainline even at the expense of long queues on entrance ramps. Waits at ramp meters of five to ten minutes were common during peak hours on some facilities, and some waits were as long as 20 minutes. As congestion and ramp waits increased in the 1990s, public frustration with the ramp metering system mounted. Refer to the following links:
Published Nov 28, 1999
http://www.startribune.com/stories/781/36320.html
Published Nov 28, 1999
http://www.startribune.com/stories/784/40437.html
C. 2000 TWIN CITIES RAMP METER STUDY
To address the frustration discussed in Section B, above, the Minnesota Legislature mandated that testing be done to assess the effectiveness of the ramp meters. This testing took place in fall of 2000, and was perhaps the single most comprehensive experiment in the history of surface transportation. Background travel data was gathered prior to the test, and then all ramp meters in the Twin Cities region were turned of for a period of eight weeks. Four interstate corridors were studied during this time, but motorists were not informed as to which corridors were being studied, because there was concern that this knowledge may affect traveler behavior.
The very extensive data generated by the test has been analyzed by two teams:
· Cambridge Systematics, Inc; and
· University of Minnesota Department of Civil Engineering.
Review a summary of the analysis, findings, and recommendations prepared by the Cambridge Systematics team at the following link:
http://www.dot.state.mn.us/rampmeterstudy/pdf/execsummary/executivesummary.pdf
Review the results of the analysis and findings of the University of Minnesota Team at the following link (Introduction, Summary, and Conclusions only):
The overall findings of the study and associated analyses were not unexpected:
· Ramp meters generally improve flow conditions on mainlines
· Ramp meters favor long commutes over short commutes
· Ramp meters improve the reliability of the interstate system (decreased variability of travel times for given trips at given times)
· Ramp meters decrease the equity of the system (fewer motorists bear greater burdens of delay)
What both sets of analysts observe is that, while ramp metering may be “doing largely what it is supposed to be doing”, the Mn/DOT should perhaps shift the operating policy balance from focusing primarily on mainline flow to reducing local impacts (operate meters less aggressively such that queues on ramps will be reduced relative to pre-test conditions). The Cambridge Systematics team recommends “a new balance” between the system users and the delays at access points, and the University of Minnesota team frames the problem as an equity issue.
Subsequent to the 2000 test, the ramp metering system in the Twin Cities has been revised to move the operational philosophy from optimizing freeway flow characteristics towards reducing waits at ramp meters (although it still is likely one of the more aggressive systems in the country). Quicker red/green cycles the meters have been established, and no waits at meters are to exceed four minutes. In addition, detectors have been installed key ramps throughout the system—when queues get to given lengths, the red/green cycle for the given ramp is quickened to clear the queue. This process feeds through Traffic Management Center’s overall data and operations management system. The rate at which the ramp is cleared is affected by the flow characteristics of the relevant mainline section.
The Traffic Management Center is a Mn/DOT operation which coordinates ITS activities and overall operations associated with the metro freeway system.
Refer to the following link:
http://www.imsasafety.org/journal/mayjun20203.htm
E. QUESTIONS/DISCUSSION
1. What does the anger with the ramp meter system suggest about how people value time in different conditions, including different driving conditions?
2. Ramp meters clearly favor longer commutes over shorter commutes. What implications does this have regarding land use planning issues and associated policy goals?
3. What is the role of public input in transportation operational decision- making--can transportation professionals devise “optimal” systems based only upon technical/analytical techniques?
4. What implications do ramp meter policies have on surface (non-interstate) roads?
5. Should transportation planners focus more on demand reduction measures and transit improvements rather than Transportation Supply Management measures such as ramp meters?
NOTE:
1Information summarized from Ramp-Metering Technology and Practice: Tasks 1 and 2 Summary, Nadeem Chaudhary, and Carroll Messer
FURTHER INFORMATION:
http://www.freedomtodrivemn.com/day_welcome.html
2002 Intelligent Transportation Systems (ITS) Projects Book
U.S. Department of Transportation
Intelligent Transportation Systems (ITS) Joint Program Office
http://www.itsdocs.fhwa.dot.gov//JPODOCS/REPTS%5FTE//13631.html
Cambridge Systematics
ENTIRE REPORT outline
http://www.dot.state.mn.us/rampmeterstudy/reports.html
Published Nov 28, 1999
http://www.startribune.com/stories/462/35174.html
Published Nov 28, 1999
http://www.startribune.com/stories/462/41423.html
Published Dec 3, 2000