Category Archives: ramp metering, capacity

Metering Motoring | streets.mn

Cross-posted from streets.mn: Metering Motoring

Metering Motoring

Ramp Meter at I-35W

 

“[If such drivers have no faster alternative route], Those are the people who I would encourage to change jobs or change houses” and “The City of Edina needs to build some arterials.”
– Mn/DOT Ramp Meter Chief Engineer (November 28, 1999 Minneapolis Star Tribune)

Ramp meters, traffic signals posted on freeway entrance ramps, seek to regulate the flow of traffic entering the freeway. They serve two main purposes; first, they limit the number of vehicles trying to merge simultaneously, smoothing traffic flow (and reducing crashes); second, they keep the total number of vehicles on the freeway trying to simultaneously use a critical bottleneck just below a threshold (capacity), so that freeway flow doesn’t exceed capacity, and thereby avoiding queueing. In and of themselves, those are both reasonable goals for managing a mature system, and most travelers readily accept traffic lights in other contexts. Yet somehow, in the Twin Cities of Minneapolis and Saint Paul, Minnesota, ramp meters became the transportation issue of 2000.

The reasons why are clear in retrospect, but may not have been in advance. As can be seen in the Figure, ramp meters were slowly deployed in the 1970s and 1980s, and became much more widespread in the 1990s. As road capacity was built out, additional roads became more and more difficult to build, not only in monetary cost, but also in political will. The leadership of the Minnesota Department of Transportation (Mn/DOT) viewed ramp meters as a way of stretching the system slightly further, eking out a small capacity improvement and a significant speed improvement at a cost much below that of adding lanes to the freeway.

Yet the Twin Cities continued to grow, as did peak hour travel demand. The primary effect of ramp meters is to move delay from the freeway to the entrance ramp. By the late 1990s, some commuters experienced long delays at some ramps, in cases upwards of 20 minutes. In 1999, Dick Day, a State Senator from Owatonna, Minnesota, a rural community outside the (metered) metropolitan area, pushed a “Freedom to Drive” package. This package called for shutting off all of the ramp meters, allowing all cars to use HOV lanes, and establishing the left lane as a passing-only lane. (Day claims to drive 70,000 miles a year, which averages to over 3 hours a day in his car – the reader can assess whether this is reasonable or hyperbole).

Day was able to obtain press for his initiative, and in November 1999, the Minneapolis Star-Tribune, the state’s largest newspaper, printed a large Sunday, front page piece on ramp meters. (The opening quote is from that article). Discussions with the engineers reveal several things. First, they were certain metering was the right thing, and they believed that shutting off the meters would be “catastrophic.” Second, they were indifferent to the fact that some drivers had long commutes so that others would have shorter commutes. They did not see ramp delay as an important metric. Rather, if the freeway flows were higher with than without meters, and at higher speeds, they knew they were reducing total delay (if more total travel is using the freeway, then there is less total travel on alternative slower routes). Third, they were highly resistant to outside analysis, probably because of distrust that the outcome would differ.

Nevertheless, to avoid the threatened shutdown, Mn/DOT commissioned three separate University of Minnesota studies to evaluate meters. One might suggest these studies were a holding strategy, essentially telling the state legislature “see we are studying this – please go away.” However, those studies did not involve shutting down the ramp meter operations, rather they would conduct computer simulations to examine operations with and without meters, compare metering approaches from a number of cities, and examine empirical data. Despite these studies, in May 2000, the Minnesota state legislature insisted on a shutdown experiment, which would last at least 4 weeks. A large consulting firm was hired to conduct the study. Many of Mn/DOT’s ramp meter engineers were excluded from the study process (their biases and lack of political acumen having been demonstrated), as were the university researchers (who were funded by Mn/DOT and therefore tainted by association). Traffic data were collected before the shut-off period, and then the meters were to be shut-off for a period of at least 4 weeks to conduct the study in October 2000. Because of weather, the study was extended a few more weeks. Due to the lack of catastrophe, the study was extended a few more, since it was clear that Mn/DOT could not return to the old metering strategy, and no new strategy was obvious. Eventually the meters were turned on (December 2000), but running at their fastest rate, so that queues would not get too long. Over time, a new strategy was developed to cap maximum waits at the ramps at 4 minutes.

Dick Day was not entirely satisfied, and Mn/DOT staff were unhappy with the shift in their worldview, but the residents of the Twin Cities seem happier with the system than before.

 

Adapted from Garrison, W and Levinson, D (2014) The Transportation Experience: Second Edition. Oxford University Press.

Further Reading:

Ramp Metering and Freeway Bottleneck Capacity

Recently published:
Zhang, Lei and David Levinson (2010). Ramp Metering and Freeway Bottleneck Capacity. Transportation Research: A Policy and Practice 44(4), May 2010, Pages 218-235.

This study aims to determine whether ramp meters increase the capacity of active freeway bottlenecks. The traffic flow characteristics at 27 active bottlenecks in the Twin Cities have been studied for seven weeks without ramp metering and seven weeks with ramp metering. A methodology for systematically identifying active freeway bottlenecks in a metropolitan area is proposed, which relies on two occupancy threshold values and is compared to an established diagnostic method – transformed cumulative count curves. A series of hypotheses regarding the relationships between ramp metering and the capacity of active bottlenecks are developed and tested against empirical traffic data. It is found that meters increase the bottleneck capacity by postponing and sometimes eliminating bottleneck activations, accommodating higher flows during the pre-queue transition period, and increasing queue discharge flow rates after breakdown. Results also suggest that flow drops after breakdown and the percentage flow drops at various bottlenecks follow a normal distribution. The implications of these findings on the design of efficient ramp control strategies, as well as future research directions, are discussed.

(pre-print available here.
(For those of you who give up hope after rejection, this paper was first submitted in 2003! My co-author earned an MS, another MS, a Ph.D. and has held two faculty positions over the duration of this article. I myself have had 3 children. After various recommendations for revise and resubmit, and changes of editor, it was lost twice by the (previous) editor of TRa (prior to the electronic submission system, which at least removes one excuse from the editor’s arsenal), and had been accepted before it was lost so had to go through re-review after the change in editorship. I think it is quite interesting and the analysis and results hold up, and so I encourage you to read it and cite it if it is relevant to your work. The final review process was relatively fast, taking 17 months from resubmission to print. And only 13 months from resubmission to online.)