Miguel Street is a winding, narrow route through the Glen Park neighborhood of San Francisco. Until a few years ago, only those living along the road traveled it, and they understood its challenges well. Now it's packed with cars that use it as a shortcut from congested Mission Street to heavily traveled Market Street. Residents must struggle to get to their homes, and accidents are a daily occurrence.
The problem began when smartphone apps like Waze, Apple Maps, and Google Mapscame into widespread use, offering drivers real-time routing around traffic tie-ups. An estimated 1 billion drivers worldwide use such apps.
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The problem is getting worse. City planners around the world have predicted traffic on the basis of residential density, anticipating that a certain amount of real-time changes will be necessary in particular circumstances. To handle those changes, they have installed tools like stoplights and metering lights, embedded loop sensors, variable message signs, radio transmissions, and dial-in messaging systems. For particularly tricky situations—an obstruction, event, or emergency—city managers sometimes dispatch a human being to direct traffic.
But now online navigation apps are in charge, and they're causing more problems than they solve. The apps are typically optimized to keep an individual driver's travel time as short as possible; they don't care whether the residential streets can absorb the traffic or whether motorists who show up in unexpected places may compromise safety. Figuring out just what these apps are doing and how to make them better coordinate with more traditional traffic-management systems is a big part of my research at the University of California, Berkeley, where I am director of the Smart Cities Research Center.
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Here's how the apps evolved. Typically, the base road maps used by the apps represent roads as five functional classes, from multilane freeways down to small residential streets. Each class is designed to accommodate a different number of vehicles moving through per hour at speeds that are adjusted for local conditions. The navigation systems—originally available as dedicated gadgets or built into car dashboards and now in most smartphones—have long used this information in their routing algorithms to calculate likely travel time and to select the best route.
Initially, the navigation apps used these maps to search through all the possible routes to a destination. Although that worked well when users were sitting in their driveways, getting ready to set out on a trip, those searches were too computationally intensive to be useful for drivers already on the road. So software developers created algorithms that identify just a few routes, estimate the travel times of each, and select the best one. This approach might miss the fastest route, but it generally worked pretty well. Users could tune these algorithms to prefer certain types of roads over others—for example, to prefer highways or to avoid them.
The digital mapping industry is a small one. Navteq (now Here Technologies) and TomTom, two of the earliest digital-map makers, got started about 30 years ago. They focused mainly on building the data sets, typically releasing updated maps quarterly. In between these releases, the maps and the routes suggested by the navigation apps didn't change.
When navigation capabilities moved to apps on smartphones, the navigation system providers began collecting travel speeds and locations from all the users who were willing to let the app share their information. Originally, the system providers used these GPS traces as historical data in algorithms designed to estimate realistic speeds on the roads at different times of day. They integrated these estimates with the maps, identifying red, yellow, and green routes—where red meant likely congestion and green meant unrestricted flow.
As the historical records of these GPS traces grew and the coverage and bandwidth of the cellular networks improved, developers started providing traffic information to users in nearly real time. Estimates were quite accurate for the more popular apps, which had the most drivers in a particular region.
And then, around 2013, Here Technologies, TomTom, Waze, and Google went beyond just flagging traffic jams ahead. They began offering real-time rerouting suggestions, considering current traffic on top of the characteristics of the road network. That gave their users opportunities to get around traffic slowdowns, and that's how the chaos began.
On its face, real-time rerouting isn't a problem. Cities do it all the time by changing the signal, phase, and timing of traffic lights or flashing detour alerts on signs. The real problem is that the traffic management apps are not working with existing urban infrastructures to move the most traffic in the most efficient way.
First, the apps don't account for the peculiarities of a given neighborhood. Remember the five classes of roads along with their estimated free-flow speeds I mentioned? That's virtually all the apps know about the roads themselves. For example, Baxter Street in Los Angeles—also a scene of increased accidents due to app-induced shortcutting—is an extremely steep road that follows what originally was a network of goat paths. But to the apps, this road looks like any other residential road with a low speed limit. They assume it has parking on both sides and room for two-way traffic in between. It doesn't take into account that it has a 32 percent grade and that when you're at the top you can't see the road ahead or oncoming cars. This blind spot has caused drivers to stop unexpectedly, causing accidents on this once-quiet neighborhood street.
The algorithms also may not consider other characteristics of the path they choose. For example, does it include roads on which there are a lot of pedestrians? Does it pass by an elementary school? Does it include intersections that are difficult to cross, such as a small street crossing a major thoroughfare with no signal light assistance?
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On top of all these problems, these rerouting apps are all out for themselves. They take a selfish view in which each vehicle is competing for the fastest route to its destination. This can lead to the router creating new traffic congestion in unexpected places.
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To compound the "selfish routing" problem, each navigation application provider—Google, Apple, Waze (now owned by Google)—operates independently. Each provider receives data streamed to its servers only from the devices of its users, which means that the penetration of its app colors the system's understanding of reality. If the app's penetration is low, the system may fall back on historical traffic speeds for the area instead of getting a good representation of existing congestion. So we have multiple players working independently with imperfect information and expecting that the entire road network is available to absorb their users in real time.
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The city engineers are also working in isolation, with incomplete information, because they have no idea what the apps are going to do at any moment. The city now loses its understanding of the amount of traffic demanding access to its roads. That's a safety issue in the short term and a planning issue in the long term: It blinds the city to information it could use to develop better traffic-mitigation strategies—for example, urging businesses to consider different work shifts or fleet operators to consider different routes.
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In the meantime, neighborhoods and citizens are fighting back against the strangers using their streets as throughways. In the early days of the problem, around 2014, residents would try to fool the applications into believing there were accidents tying up traffic in their neighborhood by logging fake incidents into the app. Then some neighborhoods convinced their towns to install speed bumps, slowing down the traffic and giving a route a longer base travel time.
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Last year, an unfortunate situation in Los Angeles during the 2017 wildfires clearly demonstrated the lack of congruence among the rerouting apps and traditional traffic management: The apps directed drivers onto streets that were being closed by the city, right into the heart of the fire. This is not the fault of the algorithms; it is simply extremely difficult to maintain an up-to-date understanding of the roads during fast-moving events. But it does illustrate why city officials need a way to connect with or even override these apps. Luckily, the city had a police officer in the area, who was able to physically turn traffic away onto a safer route.
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These are mere stopgap measures; they serve to reduce, not improve, overall mobility. What we really want is a socially optimum state in which the average travel time is minimized everywhere. Traffic engineers call this state system optimum equilibrium, one of the two Wardrop principles of equilibrium. How do we merge the app-following crowds with an engineered flow of traffic that at least moves toward a socially optimized system, using the control mechanisms we have on hand? We can begin by pooling everyone's view of the real-time state of the road network. But getting everybody in the data pool won't be easy. It is a David and Goliath story—some players like Google and Apple have massive back-office digital infrastructures to run these operations, while many cities have minimal funding for advanced technology development. Without the ability to invest in new technology, cities can't catch up with these big technology providers and instead fall back on regulation. For example, Portland, Ore., Seattle, and many other cities have lowered the speed limits on residential streets to 20 miles per hour.
There are better ways. We must convince the app makers that if they share information with one another and with city governments, the rerouting algorithms could consider a far bigger picture, including information from the physical infrastructure, such as the timing schedule for traffic lights and meters and vehicle counts from static sensors, including cameras and inductive loops. This data sharing would make their apps better while simultaneously giving city traffic planners a helping hand.
As a first step, we should form public-private partnerships among the navigation app providers, city traffic engineering organizations, and even transportation companies like Uber and Lyft. Sharing all this information would help us figure out how to best reduce congestion and manage our mobility.
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A very interesting read on how navigation apps work and how they're giving rise to some unexpected problems for city residents and planners. I've tried to capture the major points here but it's worthwhile going through the entire article.
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