2. Background

The first form of ramp metering consisted of a police officer manually directing the traffic flow from a ramp on the Eisenhower Expressway in Chicago, and it was deployed in the early 1960s. Ramp metering then spread on an experimental basis to other major cities in the United States (e.g., Detroit, Los Angeles, and Minneapolis–St. Paul) and became more and more tested and advanced. It eventually began to also be adopted in Europe and Oceania. Ramp metering began spreading in Europe during the 1980s, when the UK installed it first on the M6 near Walsall (later updated) in 1986. The Netherlands introduced ramp metering in Amsterdam in 1989, and it then spread to other cities like Rotterdam and Utrecht, and Paris (Figure 2) with more recent projects like the Praktijkproef Amsterdam started in 2013. In Germany, ramp metering can be found on the Autobahn around Hamburg, Munich, and several other areas in the Rhine-Ruhr region. In Europe, international research on the subject has been encouraged in the last two decades, with dedicated funding for projects like the Network of Excellence for Advanced Road Cooperative Traffic Management in the Information Society (NEARCTIS) and the European Ramp Metering Project (EURAMP).

Figure 2. Ramp metering deployment on Paris peri-urban motorways (AA2 Action, France) ^[3].

In Australia, this technology is widely used in Melbourne. The first form of similar traffic management was deployed in 1971, evolving into more recent solutions like integration with the Sydney Coordination Adaptive Traffic System (SCATS) ^[4]. Ramp metering eventually landed in New Zealand in the early 1980s, being gradually implemented over the years and representing the most extensive ramp metering system in the Southern Hemisphere ^[5]. The application of ramp metering to diverse cities and countries has significantly contributed to the evolution of this technology, as adaptation to the diverse city layouts and road setups across the globe is generally necessary (the differences between American metropolises and European cities, with their different road systems and extensions, immediately come to mind). Many research projects revolve around elaborating and improving algorithms, some of which will be reviewed in the following chapters.

Despite the benefits of ramp metering, its diffusion is often hindered by various kinds of challenges. Agencies commonly complain about the lack of political and financial support; the financial aspects include the costs for installation and maintenance, which includes staff training. Moreover, funding requests might compete with other projects having higher priority. Feasibility studies followed by cost–benefit analysis and performance metrics can demonstrate these systems’ economic validity, proving the advantages they can bring while reducing the expenses caused by their absence (e.g., costs related to accidents and freeway queues). Other local agencies (including the public) might contend the application of ramp metering due to a negative perception arising from a lack of both technical knowledge and awareness of the benefits. Implementing communication plans and information campaigns can be effective solutions, helping to gather consent and let the public get familiar with ramp meters. The geometry of existing ramps, in many cases, is not immediately compatible with ramp metering. It will not allow for easy installation, with common problems being inadequate acceleration lengths, ramps being too short or too close to each other, and visibility problems related to limited sight distances (vertical and horizontal spotting sights). If funding for infrastructure improvement schemes is available, this could represent an excellent opportunity to update the most strategically appropriate ramps with refurbishment and modernization work. Where this is not possible—or can be performed in conjunction—research for new solutions and technical approaches to be implemented should be done.

3. Economical Aspects and Critical Issues

As shown above, ramp metering represents an efficient technique that, with minimum economic impact, can affect a traffic volume management strategy positively on the freeway so that general traffic flow can occur without creating disease in minor roadways ^[6]. However, like any other engineering artifact, it is necessary to analyze the economic aspects deeply. Two main phases are fundamental: deployment and maintenance. From an economic point of view, they represent the immediate impact. The main obstacle of ramp metering system implementation regards the institution’s interests. With recent innovations, such as autonomous vehicles and smart cars ^[7], they are more likely to allocate funds to other transportation infrastructure upgrades concerning smart roads environments (SREs) ^[8] and intelligent transportation systems (ITSs), considering that future perspectives of communities are leading to the gradual implementation of smart cities ^[9]. Therefore, necessary investments for ramp metering deployment are first subjected to an in-depth analysis of the art’s context state and where it will be implemented. Of course, the analysis of traffic flow will be fundamental. However, it will not represent the main aspects, considering that ramp metering proposal congestion phenomena and capacities would have been detected. Ultimately, the investment quantity will be linked to the required adaptions of existing ramps being subjected to queues and platoons more frequently. It is possible to provide a piece of general information about investment and cost levels concerning ramp metering approaches and the control strategies, shown in Figure 8, and based on ^[10] if the expected investments levels are low, medium, or high and if the cost types are affected.

Figure 8. General comparison of control strategy cost levels ^[10].