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Ramp Meters

What they are, why we have them and what we've learned

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Twin Cities Metro Area Ramp Meter Study FAQs

 

Feb. 2001 FAQs

 

 

The ramp meter study has concluded. What were the results?

Field traffic data collection results presented to MnDOT by Cambridge Systematics reveal that without ramp meters there was:

 

Market research data collection results showed a number of changes in attitudes among area travelers that occurred once meters were shut off.

 

The benefit-cost ratio showed that when the costs of the entire congestion management system (including changeable message signs, traveler information, etc.) is factored in, the benefit/cost ratio for ramp metering is 5:1. When ramp meter benefits are compared to only those costs directly associated with ramp metering, the benefit/cost ratio is 15:1. Either way, the report concludes that ramp metering is a cost-effective investment of public funds for the Twin Cities area.

 

What is MnDOT's reaction to these results?

According to MnDOT Commissioner Elwyn Tinklenberg, "The study served two important public purposes. It thoroughly documented the benefits resulting from ramp metering to traffic operations and related factors, and it demonstrated the need for Mn/DOT to adjust its approach to ramp metering in a way which will optimize benefits while conforming to public expectations. In other words, MnDOT needs to balance the efficiency of moving as much traffic during the rush hours as possible, consistent with safety concerns and public consensus regarding queue length at ramp meters."

 

What does the future hold for ramp metering?

MnDOT remains committed to continued evaluation and experimentation with the ramp metering system, in close consultation with concerned stakeholders and the public. The Department also remains committed to strategically addressing issues of growth, congestion, capacity expansion and transportation choice in the metro region. MnDOT recognizes that ramp meters alone are neither the only problem nor the only solution to these major issues. In the meantime, MnDOT is moving forward to address several recommendations contained in the report. Those include:

 

MN/DOT will continue to rely on outside expertise to help it develop and implement technically sound ramp meter strategy, policy and modification evaluation plans and to review and validate the results of those evaluations. Travel time, safety, travel time reliability, throughput, and customer satisfaction are among the measures of effectiveness expected to be regularly monitored and tracked.

 

Will the current modifications continue and will the impacts of those modifications be studied?

Meters will continue to be on for a shorter period of time during rush hours. Meters will continue to change more quickly from red to green and several meters will remain off. MnDOT will continue to evaluate the impacts of the current modifications, with third party oversight and guidance provided the Cambridge Systematics Consulting team and various stakeholders. Those modifications include:

  • Limited hours of ramp meter operation (from 6:30 – 8:30 a.m. and 3:30 – 5:30 p.m). Although the meters can come on between these hours, ramp meters are traffic activated and will come on only if certain congestion thresholds are met. Prior to the shutdown, meters could operate between 6:00 – 10:00 a.m., and 2:30 – 7:30 p.m.
  • Faster ramp meter cycle times. Metering will range between 2 and 15 seconds per vehicle, depending on conditions.
  • Fewer operating meters. During the morning hours, at least 60 of 273 operating meters will remain off (flashing yellow), and at least 63 of 329 operational meters will remain off (and flash yellow) in the afternoon.

 

How did MnDOT select the meters that will remain off?

Data collected before the meters were turned off and preliminary findings indicated that some meters could remain off. MN/DOT’s Commissioner made the decision to leave some off based on expert third party advice and input from technical experts, community leaders, and others.Meters that will remain off during the modification period were selected on one of four possible criteria:

 

Why is my ramp meter off (or on)?

The actual turn-on of meters is based on a set of complex mathematical formulas called algorithms which take into account various congestion level thresholds – including volume data (information on the numbers of vehicles actually traveling through the system). These data are gathered by 3700 loop detectors (sensors) located in the pavement throughout the Metro area. These sensors are polled once every 30 seconds by a computer at the Traffic Management Center which then automatically either activates a meter, or determines it should remain in a flashing yellow mode.

 

How many metered ramps are actually active during the day? I've seen different numbers reported.

System-wide there are 430 metered ramps. However, in the morning when traffic is primarily "inbound" or moving suburb to suburb, a total of 213 ramps can be turned on. In the evening, when traffic is primarily "outbound", a total of 266 metered ramps can be activated to flash green, yellow, and red.

 

Why are meters flashing yellow when the mainline is stop and go?

During those hours when ramp meters can be activated (6:30 to 8:30 in the morning and 3:30 to 5:30 in the evening), they will do so based on volume/congestion thresholds which when met trigger the start-up of a meter. These thresholds are based on the number of vehicles entering the freeway in a particular area (or zone) and the number of vehicles already on the freeway in that same zone. (To help it better manage the freeway network, MnDOT has divided it into 93 zones, each between three and five miles long).

 

Under the recently enacted ramp meter modifications, when a zone becomes so congested that metering would no longer be helpful to the efficient movement of traffic through it, the zones meters will simply continue to flash yellow. If congestion thresholds in a specific zone are met prior to the start of ramp meter operating hours, the meters in that zone will continue to flash yellow throughout the rush hour.

 

How were metering rates selected? How was this different than before?

Prior to the ramp meter shutdown, and prior to the recently enacted modifications, there were six different metering rates that could be applied at any given meter, at any given time during the morning and evening rush hour. The range of red times for each of those rates – excluding the 2-second yellow/green time was:

 

Rate 1 = from 0.1 to 11.3 seconds
Rate 2 = from 0.1 to 13.4 seconds
Rate 3 = from 0.1 to 16.2 seconds
Rate 4 = from 0.1 to 18 seconds
Rate 5 = from 0.3 to 18 seconds
Rate 6 = from 0.6 to 18 seconds

 

The modifications currently under study call for metering rates to be capped at Rate 2.

 

Rate 1 = red time ranges from 0.1 to 11.3 seconds
Rate 2 = red time ranges from 0.1 to 13.4 seconds

 

Note: The actual red time for a specific ramp is based on the number of vehicles that use that ramp in a given hour. The higher the volume on the ramp the lower the red time used for the ramp.

 

MnDOT’s decision to "cap" metering rates at rate 2 reflects input it received from numerous sources, including daily rush-hour commuters, the consultant team working on the study, the ramp meter shutdown study’s Citizen’s Advisory and Technical Committees, and traffic management and engineering experts from within the Department. It was clear, based on all that input, that MnDOT’s ramp metering strategy and policy could not be the same as it was during pre-shutdown conditions.

 

How does MnDOT's Traffic Management Center determine which rate a meter runs at?

Whether a meter runs at rate 1 or 2, and just how much "red-time" a traveler will experience at any given meter during actual metered/operating hours, is based on a variety of factors programmed into custom software written for MnDOT’s Traffic Management Centers computers. (see previous questions regarding metering rates and why ramp meters turn-on or stay in a flashing yellow mode). Generally, The actual red time for a specific ramp is based on the number of vehicles that use that ramp in a given hour. The higher the volume on the ramp the lower the red time used for the ramp.

 

In some instances, I've seen congestion happening prior to the designated hours of operation for the meters. Why can't you turn the meters on earlier than 6:30 in the morning or 3:30 in the afternoon?

During the course of this legislatively mandated study, it became clear that an all or nothing approach is not the best. Market research showed that 20 percent of commuters want the meters left off, roughly 10 percent of commuters want the ramp meters on as before, and about 70 percent want the ramp meters modified. (They want fewer meters, shorter operating hours and faster lights). The current modifications are a first cut at finding a balance between safety, system efficiency and public consensus. The impacts of these modifications will be monitored, and adjustments made as traffic patterns change and congestion levels continue to grow. For now, hours that the meters can come on are 6:30 to 8:30 in the morning and 3:30 to 5:30 in the evening.