Why Minibusways Can Work

Autonomous vehicles (AVs) stand to revolutionize transportation, economies, and cities. They will make better use of road infrastructure, reduce collisions, help people get around, make goods more affordable, and reduce the need for parking. They will also draw riders away from traditional transit systems, which have been slow to introduce driverless technology and offer more limited mobility options. In this piece, I argue that two keys to maximizing the benefits of AV technology are using it to replace public transit lines and using dynamic tolls to allocate road space. These two pieces together can create a new kind of transportation corridor that I dub the Minibusway.

The claim that autonomous vehicles could replace subways, buses, and rail lines has been the subject of heated debate. This debate has often revolved around the assumption that these autonomous vehicles would be personal cars with one or few occupants. It is clear that driverless cars would not be able to deliver enough peak capacity to carry as many riders as a subway line; it’s geometrically impossible. A lane carrying 3,000 autonomous vehicles per lane per hour with an average 1.5 passengers produces a capacity of 4,500 people per hour. Those 4,500 people could fit into just three New York City Subway trains with room to spare.

However, the revolution in AV technology is about more than just private driverless cars. In dense cities especially, vehicles of all sizes will help people get around. If those 3,900 AVs are instead carrying 30 people each, then all of a sudden, they can carry a lot more people than a subway line. This is why I propose converting current transit corridors into minibusways once the technology is ready.

This matters because AVs are about much more than maximum capacity; they’re about helping people and goods get around safer, faster, more flexibily, more consistently, and more affordably.

This piece will discuss how minibusways would work in a world where AV technology is advanced and reliable. At present, there is still a ways to go, but train lines may be one of the first places AVs could work well, as they’re designed to be separated from human-driven vehicles, pedestrians, and the kind of novel situations that AVs struggle with. It’s worth ruminating on how we could best use this technology to improve cities can help us prepare for the future as well as understand current transportation challenges.

Getting Road Prices Right

Small, driverless buses A big, driverful bus

Minibusways use dynamic tolling (a form of decongestion pricing) to keep traffic moving. Under dynamic tolling, the toll on a particular road varies based on how many people want to use that road. When the toll gets higher, more people decide not to travel on that road, which keeps it from getting congested. The toll is designed to be as low as possible while preventing congestion. This way, traffic is always moving smoothly.

Dynamic tolling can be hugely beneficial today, and it will only grow more important when AVs become widespread. If all roads are free, there will be little to stop AVs from circling the streets of busy cities in between trips rather than paying to park. Moreover, AVs will make travel a lot more comfortable, as everyone, not just those in transit, taxis, and rideshares, will be able to spend their time doing other things than driving, and this will both increase how much people travel make people more willing to tolerate delays.

Though AVs will increase road capacity by running closer together and integrating with traffic control, they might lead to worse traffic in dense cities as more people decide to travel alone in cars. Dynamic tolls on minibus would disrupt this, encouraging travelers to take less busy routes, incentivizing people to keep empty vehicles off of busy roads, and encouraging people to share rides. The best system will involve dynamic tolls on minibusways, highways, and throughout a city’s entire road network.

On busy routes, people will gravitate toward sharing rides because this allows you to split the cost of the toll. A $20 toll is a pain if you’re driving alone, but it’s no big deal if you can split it 20 ways with your fellow passengers on a minibus. This doesn’t work with traffic delays on free roads; you’ll get stuck in a 30-minute slowdown whether you’re carpooling or not.

In this system, the toll will in large part determine the mix of vehicle types you see on a given route at a given time. For the most popular journeys, vehicles will tend to carry more people, as high tolls will encourage more sharing and hence, higher-capacity vehicles. On less popular routes, people will be more apt to spring for a more space and more convenient pickups, be that in a roomier minibus, a 7-seat SUV, or a private car. However, in either scenario, people’s choices will vary. Everyone will have the choice to pay for as much space and convenience as they want. The right mix of vehicles will emerge based on these preferences.

While the price system might raise concerns about equity, it’s important to remember that every vehicle would travel at the same speed and that these tolls would be government revenues, meaning that the $50 a rich person pays on a private ride would go toward either lowering taxes (e.g. local sales tax) or providing government services.

How Minibusways Would Work

Whenever possible, minibusways, high-capacity roadways to replace train lines and busways, should have separate stopping and passing lanes at stations for traffic in each direction. This would allow for much quicker journeys and dramatically higher throughput because minibuses wouldn’t have to wait behind each other. The through lanes would function like highway lanes, sustaining a consistently high, fast flow of vehicles.

Making Fewer Stops

To use a minibusway, a traveler would use a mobile app or a touchscreen kiosk at the minibus station to select their destination before they get on a minibus. The system would direct the traveler onto a specific minibus based on the destination they selected. This concept already exists in some transportation systems, such as West Virginia University’s Morgantown Personal Rapid Transit, which is described in the video below. It allows people with the same destination to cluster into the same vehicle, which then skips most or all other stops on its way to that destination. Three key differences between PRT and the minibusway concept are that PRT vehicles move much slower, are typically smaller, and work only on special guideways, as opposed to any paved road.

When someone inputs their destination, their screen would indicate which minibus to board. The screen might say, “Board minibus B3, which is due to arrive in 2 minutes,” and the traveler would wait to board a minibus displaying “B3.” It’s similar to to the “destination dispatch” system used for elevators in some office buildings:

As long as there is a decent level of demand on a minibusway, each vehicle will function as an express service, only stopping at a few stations over the course of an entire line. , As with elevators, you might share an AV if you’re traveling at a busy time, but get one to yourself when things are quiet.

The system offers the spatial efficiency of transit but much greater customization and flexibility. Say 20 people in Tuckahoe, NY are all commuting to Grand Central Terminal in Manhattan at 8:00 a.m on a weekday. Currently, the have to board a train that then makes 3-11 intermediate stops because there aren’t enough passengers at Tuckahoe to warrant getting an entire train to themselves. However, if this were a minibusway, they could all board a 20-seat minibus heading directly to Grand Central without no intermediate stops.

Offering Fast, Frequent Service All Day

Minibuses could also operate much more frequently than trains. Trains typically require at least a driver (and sometimes conductors as well), and transit agencies want to save money running fewer trains with more passengers per train. With AVs, that’s not a concern. Thus, instead of one train every 10 minutes, a station could see one minibus stopping every minute. More frequent service makes journeys faster, and it eliminates the risk of missing a train/bus and waiting ages for the next one, thus making transit a lot more reliable and a lot more useful.

Autonomous minibuses would function as part of a synchronized platform, giving people more choices in how to travel. You might ride your own car over to the minibusway (or highway) before switching to a high-capacity minibus to take you downtown for work in order to avoid a paying an expensive toll. The AV network would know when you get out of your car and ensure that a minibus pulls up within a minute or two to pick you up.

Some minibusways might look like traditional highways, with ramps that let vehicles travel on and off freely. For others, especially minibusways that were former subway lines, it might not be practical or desirable to build a bunch of ramps in dense urban environments. In these cases, people would either walk or take an AV to the station before walking down (or up) to the platform to board, as they do today.

Estimating the Capacity of a Minibusway with Passing Lanes

So, what would the capacity of a four-lane minibusway look like?

In Autonomous Driving: Technical, Legal, and Social Impacts, Professor Bernard Friedrich uses a formula to derive how much additional capacity AV technology would create on a highway. Applying his formula to autonomous minibuses yields an estimate of 4,201 AMs per lane per hour. If AMs are designed like coach buses, with every passenger getting a seat, each 20-foot minibus could carry about 25 people, for a total of 104,193 passengers per lane per hour. If instead these AMs were designed like subway cars, with bench seating and lots of standing room, each could hold about 64 people, for a total of 268,660 passengers per hour. By comparison, New York City’s 4-track Lexington Avenue Line (4/5/6 trains) can carry theoretical maximum of 93,432 passengers per hour in each direction, or 65% less than this sample minibusway.

3. When There’s No Room for AVs to Pass Each Other

Like a Train but Different

As I described above, minibusways work best when vehicles can pass each other. However, they can still have a lot of benefits even on corridors where that isn’t possible. On these corridors, the minibuses would operate in tightly packed caravans, like train cars but not physically connected. They’d still be able to move more people than traditional trains because each caravan could travel very close to the one ahead of it.

Trains are massive and heavy, and their steel wheels don’t grip as well as rubber tires, so it takes them a long time to go from full speed to a dead stop. Thus, transportation agencies install signals that keep trains far apart from each other so that if one train stops suddenly, the one behind it can brake in time to avoid a collision. AVs are much smaller, and their onboard computers can apply the brakes very quickly after detecting another vehicle slowing down. Thus, they’re only constrained by how long each caravan has to spend stopped at a platform (specifically, the busiest platform on the line). Let’s assume this is about 40 seconds, which is the case for many trains. In that case, 90 minibus caravans, the equivalent of 90 trains, can run per hour (3600 seconds/40 seconds) in each direction. By comparison, London’s most frequent Underground line achieves 36 trains per hour.

Minibusways still have higher capacity than trains, even without passing lanes: 110,000-138,000 passengers per hour versus 40,800 passengers per hour on New York City’s L Train.

Moreover, minibus caravans could shrink and expand based on the time of day. Each caravan might have 12 minibuses during rush hour and just 2 minibuses at mid-day but operate just as frequently. Train operators tend to stick with just one size of train all day long, which often means wasting energy pulling mostly empty cars outside of rush our.

In Conclusion,

A minibusway offers higher capacity than a train line while providing a better network, faster journeys, lower operational and capital costs, and greater flexibility. Rather than reinventing the wheel, it combines a number of features from existing transportation technologies:

It allows people to cluster into vehicles based on their destination like WVU’s PRT system does.

It responds to demand like rideshare platforms do.

It can support a continuous flow of traffic like grade-separated highways do.

It uses high capacity vehicles that can run on normal streets like buses do.

It has even higher capacity than than trains.

When paired with a network of AVs on surface streets, it can ensure that everyone is dropped off within a short walk of their destination like taxis do.

To function well, dense cities don’t need trains. They don’t need traditional buses. They do need ways for a lot of people to move quickly and safely through a limited amount of space. Trains and buses do that job well in some cities, but if and when the technology is ready, autonomous minibuses will do it better.

For more, see Dynamic Tolls Promote Carpooling, Eight Reasons Autonomous Minibuses Could Replace Subways and Urban Rail,Where Autonomous Vehicles Could Start Transporting a Lot of People, Chicago Autonomous Transit, New York City Autonomous Transit, and Notes and Appendices.

One Last Thing: Some boosters of autonomous vehicles are so enthusiastic about the technology that they wheel around televisions showing groups of people footage of these vehicles in action. They call themselves the AV club.