Playing the slots – revolutionising road intersections

12 September 2016



Slot-based intersections in concert with self-driving cars could make cities more liveable by doubling road capacity, reducing emissions and improving safety. Elly Earls asks Carlo Ratti, the director of the Senseable City Lab at MIT, whether traffic lights are on the way out and what our commute to work might look like in the future.


Imagine a world where your car drives you to work – giving you precious extra time to put those final touches to your presentation – drops you off and goes to pick up your neighbour for their daily commute, all without stopping at a single traffic light – because that’s not how intersections work anymore. Not only that, the landscape sliding by outside your window is no longer made up of endless rows of concrete car parks, but green public spaces, as the efficiency of the city of the future has slashed the space needed for parking by thousands of square miles. Oh, and your journey is significantly more fuel-efficient too.

According to Carlo Ratti, the director of the Senseable City Lab at the Massachusetts Institute of Technology (MIT), a multidisciplinary research group that studies the interface between cities, people and technology, this scenario is not only possible, it’s closer than we think.

The first traffic lights date back to over a hundred years ago – a great solution at the time for the increased number of vehicles on the roads. Yet, despite huge advances in IT and automotive technology, they’ve evolved little since. This is something Ratti and his colleagues at the Swiss Institute of Technology (ETHZ) and the Italian National Research Council (CNR) hope to change.

“Our thought is that, with the advent of distributed urban IT and autonomous cars, we could imagine new systems that would allow us to make better use of our roads and our time,” he says.

And they’ve already developed a solution – slot-based intersections, which, according to their research, could significantly reduce delays, make traffic patterns more efficient and slash fuel consumption.

“An intersection is a difficult place because you have two flows competing for the same piece of real estate,” Ratti explains. “Our idea is based on a scenario where sensor-laden vehicles pass through intersections by communicating and remaining at a safe distance from each other, rather than grinding to a halt at traffic lights. In other terms, you move control from the traffic flow level to the vehicle level. Doing that, you can create a system that is much more efficient, and can significantly reduce queues and delays by ensuring the vehicles get to the intersection exactly when they have a slot.”

"Of course, transitioning from a traffic light-based system to a slot-based strategy would be a huge shift for cities and one for which the introduction would entirely rely on autonomous cars becoming commonplace."

Slot-based intersections are similar to the slot-based management systems used for air traffic control. Upon approaching an intersection, a vehicle automatically contacts a traffic management system to request access. Each self-driving vehicle is then assigned an individualised time or ‘slot’ to enter the intersection. Results from the team’s research show that by transitioning from a traffic light system to a slot-based strategy, complete with adaptive vehicle platooning, capacity could be doubled. For example, an intersection could handle twice the number of vehicles as it can now, and delays would be significantly reduced.

Furthermore, as stop and go would largely be avoided, emissions caused by acceleration and deceleration cycles would be lower. Due to the flexibility of slot-based intersections, pedestrian and bicycle crossing could also easily be accommodated. Interestingly, the optimal strategy defined in the team’s paper also leverages the ‘slower-is-faster’ effect. In other words, a slower speed results in a faster overall journey, which has obvious safety implications.

While further research would be needed to scale up the team’s analysis to a more complex network of road intersections, they are keen to get their idea out into the public domain as soon as possible. Their findings, they believe, could provide transportation engineers and city planners with crucial insights as they prepare to manage the transition towards a more intelligent urban transport infrastructure.

Self-driving cars: decades away

Of course, transitioning from a traffic-light-based system to a slot-based strategy would be a huge shift for cities and one for which the introduction would entirely rely on autonomous cars becoming commonplace; something Ratti is optimistic, yet realistic, about.

“Slot-based intersections will take some time to be fully deployed, because we need a certain level of intelligence in every car, but I do think that they will be a natural consequence of the looming driverless revolution,” he notes, adding that as cars are already able to self-drive today, there would not be any need for major infrastructural changes. “This will speed up adoption.”

But how fast could this adoption feasibly be? Yes, driverless cars already exist, and yes, more and more automotive companies are testing increasingly advanced autonomous driving technologies, but self-driving cars becoming the norm? The consensus is that this is still decades away.

That said, it’s certainly on its way. A forecast by IHS Automotive, released in June, predicts that, by 2035, 21 million fully and semi-autonomous vehicles will be sold globally, with the US expected to lead the way. US autonomous deployment will start with several thousand vehicles in 2020 and then grow to nearly 4.5 million vehicles by 2035, the report found. The forecast also took into consideration R&D announcements and collaboration projects under way, the regulatory hurdles the industry is likely to face and consumers’ views of self-driving cars,

“It’s a long timeline; nonetheless, it’s something we expect to work out,” said IHS Automotive’s principal analyst Jeremy Carlson. “There’s opportunity here for the government, for cities, however they look at this.”

Already, the concept is being tested on a small scale around the world. The University of Michigan’s Mcity, part of the institution’s Mobility Transformation Center, for example, is a 32-acre full-scale simulated real-world urban environment that has been designed to test every possible driving scenario to evaluate connected and autonomous cars. It also predicts the effect these vehicles will have on the economy and society. Earlier this year, Mcity teamed up with Ford to conduct what it believes is the industry’s first test of autonomous vehicles in wintry conditions.

Elsewhere, a South Korean university is testing a sedan called Snuber that can pick up and transport passengers without a human driver, while US-based automated vehicle start-up Varden Labs has been loaning driverless shuttles to various college campuses to give students rides to class. Ratti believes slot-based intersections could be tested very quickly in spaces like Mcity.

An omnidisciplinary approach

Technology is by no means the only hurdle that will be faced en route to Ratti’s ‘looming driverless revolution’.Getting the gamut of stakeholders on board – from industry to city planners, designers, architects and engineers – will be the only way the research team’s new traffic flow concept can ever realistically be realised. This is precisely the idea of MIT’s Senseable City Lab, which brings together experts in fields as diverse as sociology and mathematics, to look not just at how traffic systems need to evolve but also at how digital technology is impacting urban living in all its facets.

“At the lab, we try to have an omnidisciplinary approach. Over 40 people, coming from all over the world, compose the team. Each researcher has a different personal history, different skills and a different cultural background. Most of them come from architecture and design, but we also have mathematicians, economists, sociologists and physicists,” Ratti explains. “I think that diversity is a really important aspect in any team activity. On this project alone, we have worked with mathematicians, physicists, planners, architects, engineers and psychologists, in view of analysing the context and developing the system.”

"MIT’s Senseable City Lab brings together experts in fields as diverse as sociology and mathematics to look not just at how traffic systems need to evolve, but also at how digital technology is impacting urban living in all its facets."

And bringing it to fruition will take further collaboration. “Most urban transformations are the consequence of a common effort of private investors, municipalities and citizens,” Ratti notes. “They need to be inclusive. This is why we are happy when our research promotes a discussion that goes beyond the academic community – that is the first step for an inclusive discussion about how our cities should be transformed.”

Ratti, though, already has his end goal in mind. “Autonomous vehicles promise to have a dramatic impact on urban life,” he believes. “First, they will allow us to use the commuting time for other activities: reading, sleeping, relaxing, kissing. But, more importantly, they can blur the distinction between private and public modes of transportation. ‘Your’ car could give you a lift to work in the morning and then, rather than sitting idle in a parking lot, give a lift to someone else in your family, or, for that matter, to anyone else in your neighbourhood.”

The lab’s research has also shown that, theoretically, a city could function with a fraction of the vehicles that are on the road today. It could even be possible to remove eight out of ten vehicles from major cities and still satisfy their citizens’ demands for mobility. “Such results are just a lower bound and will depend on how willing we are to share cars,” Ratti admits. However, he is confident that there will be fewer cars on the road and, as a consequence, large parking areas could be transformed into green, public spaces. “At the moment, parking infrastructure is so pervasive that for every car in the US, there are approximately three non-residential spots. This amounts to 5,000 square miles, which is an area larger than Puerto Rico.”

While Ratti’s vision might sound ambitious and is certainly decades away, he and his collaborators have made a valuable first step towards realising it by proving – in theory at least – that slot-based intersections in concert with self-driving cars would not only make traffic flow more efficient, they could also improve safety and reduce emissions, ultimately making cities more liveable.

Director of MIT Senseable City Lab, Carlo Ratti is an architect and engineer. He practises in Italy and teaches at MIT. Ratti has co-authored over 250 publications and holds several patents. His work has been exhibited in several venues worldwide, including the Venice Biennale, New York’s MoMA and London’s Science Museum.


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