The Traffic Signal Knows You’re There

Making intersections smarter and cyclists safer


The fact that every busy intersection in an American city doesn’t see a dozen horrific collisions a day is, when you think about it, kind of a miracle.

Each one of them is a confined space, after all, that ushers thousands of massive vehicles through per hour, often operated by barely-trained, distracted amateurs, separated by just a foot or two of space. Throw in pedestrians, bicycles, trucks, buses and the occasional hyper-aggressive kid in an extended-cab pickup, and it’s a genuine wonder they never even touch. Well, almost never.

We certainly don’t have evolution to thank for this feat of communal avoidance. For 99+ percent of our history as a species, the only time a human ever exceeded running speed was while galloping on a horse or falling to his death, and the onset of motorized travel was so rapid that our senses have struggled to catch up ever since. The design of our roads, and especially our intersections, is testament to this. We clear them of obstacles, smooth them over with asphalt, but especially, we fill them with information—stripes, signage, three-color traffic signals, and very recently, tiny blue LEDs that do nothing but turn on and off.

These blue lights — an experiment in bike-specific infrastructure being conducted in just two US cities—are trying to prove out a bold hypothesis: When we know we’re being witnessed, even by a tiny mechanical device, our behavior changes for the better. The results could fundamentally alter the way different modes of transit communicate with the street, and with each other.

A modern intersection is so crammed with information exchange that it’d be just as accurate to call it a conversation — not a casual chat between friends, but a massive, complex and tightly structured discussion. I tend to think of a congressional debate, or a medieval monarch holding court. The petitioners arrive in droves, sort into groups of common intent, gain recognition, and receive judgement.

Like a debate or royal court, the intersection must accommodate a variety of participants with diverging goals, and most crucially, make their interactions predictable. When stakes are high and emotions fraught, predictability is what keeps us safe. So besides letting its petitioners know where to be and when to go, the intersection must also reinforce the bargain that underpins the conversation: follow the signals, be predictable, and you’ll get where you need to go. As the streetscape expands to include greater volumes of walkers, buses, streetcars and, especially, bike riders, getting universal buy-in on that bargain is no trivial task.

The components of an intersection give the conversation voice. Traffic signals, signs and street markings are how it talks. Timing circuits, beg buttons, traffic cameras and sensor loops are how it listens. Those last two are among the latest additions to the modern intersection, which is interesting because they’re also some of the least visible. It’s as if we realized at some point, a few decades back, that there was only so much you could achieve by yelling instructions at people, and that efficiencies could be gained by listening, and learning what they were up to.

Credit: U.S. Department of Transportation

So we cut circular, rectangular and diamond-shaped grooves into the asphalt, filled them with coils of wire, ran small currents through the coils, and then waited for something conductive to come along and disturb the magnetic field around them. This is how the million-plus traffic sensor loops buried in American asphalt work, through Faraday’s Law of Induction. Push a piece of conductive metal—like an engine block or a piece of sheet steel—through a magnetic field, and it induces a current in the wires that created the field. The math on this is pretty straightforward: a bigger conductor induces a bigger current. A typical car is 3000+ pounds of conductor, so its presence is instantly heard, and the traffic cycle adjusts to accommodate.

Left: Schematic design of loop detector design Right: Physical representation of loop detector. Source: U.S. Department of Transportation

For those walking or riding bikes, the shout is reduced to a whisper, and as in any conversation, those with quiet voices must speak with greater delicacy. For pedestrians, there’s a dizzying array of beg buttons, but these rarely give adequate feedback, and are often unattached to the traffic circuit. This breeds skepticism, and is part of the reason “jaywalking” is nearly universal.

Credit: Jonathan Maus, BikePortland

For bikes, there’s a metal frame and a pair of wheels to trip the sensor loop, but only 20 or 30 pounds of it—less if you’re on a pricey carbon frame—so to induce a large enough current, you must line up precisely with the loop’s edge. In a few cities, small stencils indicating ideal placement have begun to appear, to the mystification of many. Along Portland’s busier bike routes, it’s not unusual to see experienced riders coaching newbies in the finer points of sensor loop identification and alignment. Contrasted with the automatic recognition a car receives, being sensed as a walker or cyclist is a learned behavior.

This dichotomy hints at how US traffic infrastructure treats different modes in general. Despite some great strides over the past decade, the majority of American streets are still designed almost entirely with cars in mind, with pedestrian and bike-specific infrastructure tacked on top. If a street were designed for bikes, on the other hand, it would have lanes that are half as wide, more Yields than Stops, signage at eye level, and speed limits that top out around 20.

This is not to say our infrastructure can’t accommodate non-drivers, only that they must demand accommodation, rather than receiving it without thinking. Making a mid-block left turn in steady traffic, for example, is a simple matter while driving a car, but a complicated multi-step process on a bike, demanding good judgement (stop in the center? pull off on the right?) and often nerves of steel. This is entirely an artifact of road design, as the rare bike-specific turn lane shows by contrast.

That gap in accommodation isn’t particularly good for predictability. There are a range of reasons why someone using the road might not uphold their end of the bargain, such as a fundamental antipathy toward being told what to do. But the most powerful and preventable reason may be the sense of not being acknowledged. A pair of pedestrians crossing in the middle of the block quietly accuses the street of not knowing or caring about their needs. As pedestrians ourselves, most of us know that mid-block jaywalking is sometimes safer, and often more expedient, than taking the crosswalk: it provides longer sight lines, and fewer threats from encroaching taxis or distracted commuters making a hasty right turn on red.

Cyclists who run red lights make a similar call, perceiving that the infrastructure isn’t particularly interested in hearing from them, and concluding they’re better off taking matters into their own hands. It’s a breach of trust that destroys predictability, causing a lot of frustration, and occasional injury or death. Current debates about the safety of urban cycling largely boil down to a split in perceived motivation, with bike riders arguing that they’re grappling with a system that wasn’t designed for them, and their critics casting them as antisocial, irresponsible brats. This tendency, to chalk up someone’s behavior to their inherent nature, rather than a response to outside influence, is called fundamental attribution error, and it shows up just about anytime a small group of people does something a larger one doesn’t like.

The straightforward way out of this dilemma is to modify the conversation that infrastructure is having, so it’s more explicitly inclusive of non-drivers.

Those pedestrian beg buttons that beep loudly when you press them are one example. So are Sensor Confirmation Indicator lights — those blue LEDs we talked about before — a recent innovation aimed squarely at urban cyclists, that’s so far shown up at just a handful of intersections in Austin, Texas and Portland, Oregon.

Credit: Jonathan Maus, BikePortland

At roughly $125 to install, a Sensor Confirmation Indicator is among the least expensive infrastructure investments a city can make. Nothing more than a bright blue LED attached to the side of a traffic signal or mounted on a light pole, it performs the most basic function imaginable, lighting up when a bike, car or other vehicle is detected by a sensor loop or traffic camera. The Indicator gives no instructions, and makes no prohibitions. It just lights up when approached. That’s it.

And yet, as someone who’s encountered such indicators, I can attest that the experience of being detected by one while biking is profound. Perhaps because so much infrastructure is so indifferent to bicycles, discovering a piece that responds to your presence feels a bit like having the bronze statue in a memorial open its eyes and greet you by name. This tiny blue light fundamentally alters your role in the intersection’s conversation, elevating you from an unwelcome nuisance to an active participant.

The intersection of Northeast Morris and Northeast Martin Luther King in Portland is part of my daily bike commute, and like most of the other eight Sensor Confirmation Indicator locations in the city, it was chosen because a busy bike route (NE Morris) crosses a major arterial road (NE MLK) there. I’ve not once seen a driver or cyclist run that light since the Indicators went in, nor at any of the other locations. According to Peter Koonce, the head signals engineer for the Portland Bureau of Transportation (PBOT), studies at Portland State University confirm that red light compliance among people on bikes has increased significantly at every one of the nine locations.

The intersection of North Interstate and NE Oregon
The intersection of N Interstate and NE Oregon. That’s I-5 to the right, and yes, a garden in the shape of a peace sign in the lower left.

Of those intersections, the one at North Interstate and NE Oregon is worth particular scrutiny. For one thing, it’s hosting an unusually complicated conversation. A one-way street carrying car traffic off the Steel Bridge meets Interstate Avenue, a major two-way arterial that feeds into a nearby Transit Center, as well as the arena where the Portland Trailblazers play. It also absorbs heavy bike traffic from three different separated bike paths that rejoin the grid at that point.

And yet, the intersection has a stellar safety record, as well as a red light compliance rate of 99.4 percent among people riding bikes, making it perhaps the most respected traffic signal in all of North America.

The Sensor Confirmation Indicator light certainly contributes to the intersection’s success, but it’s actually a small part of the story. The bike traffic that flows onto the road there, for example, gets its own phase in the signal cycle, allowing it to cross diagonally from path to on-street bike lane in a single motion, an unusual movement that’s clearly indicated by pavement markings and dedicated signage. This diagonal phase is controlled with a traffic signal using bike-shaped stencils over the red, yellow and green lights, an innovation that’s been embraced in several US cities, despite appearing nowhere in the AASHTO “Green Book” that governs most American street design. And the blue LED mounted next to that signal is linked to a small sensor loop buried not in the asphalt, but in the sidewalk ramp where bikes naturally queue up. It sounds like a lot of fussy detail, and it is, but no more than what’s already been installed for cars at thousands of comparable intersections.

Despite the best efforts of engineers, traffic cops and conscientious road users, things still go wrong on American roads far too often, costing us over 30,000 lives a year, to say nothing of the injuries and the monetary loss.

Traffic deaths weigh heavily on the US compared to other wealthy nations, and the culprits are many. Our licensing standards are laughably low, criminal prosecution of aggressive drivers is scant, and the structure of our cities encourages average citizens to drive vast distances as a matter of habit. To think that a cheap technological fix like a blue LED could seriously impact this tragic status quo is naive at best.

Yet things are getting better. It’s safer to travel on a US street now than at any point since we started keeping records, regardless of which mode you’re using. We owe a lot of this improvement to better safety features in cars, and to more skilled emergency medical care, but we also owe it to better designed streets. Much of what makes an urban street look different than it did 10 or 20 years ago stems from an improved understanding of why people behave the way they do. We once relied on speed limits and radar guns to slow cars down; now we know that a curb bumpout or traffic circle can do the same job more effectively. Broadly speaking, we’ve learned that our actions are largely (though not entirely) a response to our environment, and many “innate” behaviors are actually design problems. The complex conversation proceeds at every intersection, and while we can’t tell people what to say, we can set the tone and choose how to respond.

This is what’s so heartening about the Sensor Confirmation Indicator, and the dozens of other small, thoughtful infrastructure elements that have begun appearing on America’s streets. The blue LED speaks with a whisper that could be easily lost in a noisy conversation, but it whispers exactly the right thing. In the middle of the chaotic miracle of modern traffic, it recognizes that the first step to predictability is acknowledgement. And rather than shout another directive, it simply offers confirmation: you are heard, you are part of this, this is for you.