Introduction

Urban and interurban motorways had been originally conceived so as to provide virtually illimited mobility to road users. The on-going dramatic expansion of car-ownership, however, has lead to the daily appearance of recurrent and nonrecurrent motorway congestions in European countries and elsewhere. Ironically, recurrent congestions of hundreds of kilometers in length reduce substantially the available infrastructure capacity at the rush hours, i.e. at the time this capacity is most urgently needed, causing delays, increased environmental pollution, and reduced traffic safety. Similar effects are observed in the frequent case of nonrecurrent congestions caused by incidents, road works etc. It has been recently realized that the mere infrastructure expansion cannot provide a solution to these problems due to economic and environmental reasons or, in metropolitan areas, simply due to lack of space.

The traffic situation on today’s motorways resembles very much to the one in urban road networks prior to the introduction of traffic lights: blocked links, chaotic intersections, reduced safety. It seems like road authorities and road users be still chasing the phantom of illimited mobility that motorways were originally supposed to provide. What is urgently needed, however, is to restore and maintain the full utilisation of the motorways’ capacity along with an orderly and balanced satisfaction of the occurring demand both during rush hours and in case of incidents. Clearly, the passage from chaotic to optimal traffic conditions is only possible if today’s “spontaneous” use of the motorway infrastructure is replaced by suitable control actions aiming at the benefit of all users. Ramp metering is the most efficient means to this end, whereby short delays at on-ramps and motorway-to-motorway intersections is the (relatively low) price to pay for capacity flow on the motorway itself, leading to substantial savings for each individual road user.

It is the aim of this paper to show, based on simple technical arguments and available field evidence, that ramp metering, if properly designed and utilized, may ameliorate dramatically the traffic conditions on motorways particularly in and around metropolitan areas.