If you live in a major city, I can take a
pretty good guess at one of your most common frustrations: traffic. In city driving, the journey is rarely better
than the destination. In most cases, we just want to get where we’re
going. Traffic is not just frustrating, but it has
consequences to the environment as well. All those idling vehicles have an impact on
air quality. When you’re stuck and sitting behind a long
line of cars, it’s easy to let your mind wander over solutions to our traffic woes. But, traffic management in dense urban areas
is an extremely complex problem with a host of conflicting goals and challenges. One of the most fundamental of those challenges
happens at an intersection, where multiple streams of traffic - including vehicles, bikes
and pedestrians - need to safely, and with any luck, efficiently, cross each others’
paths. Over the years we’ve developed quite a few
ways to manage this challenge of who gets to go and who gets to wait, from simple signs
to roundabouts, but one of the most common ways we control the right-of-way at intersections
is the traffic signal. I’m Grady and this is Public Works, my video
series on infrastructure and the humanmade world around us. This video is sponsored by Squarespace. More on that later. There are a lot of good analogies between
cities and human anatomy, and roadways are no exception. Highways are like the aorta with a high capacity
and single major destination. Small collector roads are like the capillaries
with not much capacity but a connection to every individual house and business. And, in between are the aptly-named arterial
roadways, the medium-capacity connections between urban centers. Rather than ramps, overpasses, and access
roads to control the flow of traffic, arterial roads use at-grade intersections through which
only a few traffic streams can pass at a time. We call this “interrupted traffic flow”
for obvious reasons. In most cases, these intersections are the
limit to the maximum throughput of the roadway. In other words, increasing the number of lanes
or the speed limit won’t have any effect on the overall capacity of the road. The only way to increase the number of vehicles
that safely travel from point A to B is to increase the efficiency of the intersection. In addition, these intersections are where
a vast majority of accidents occur. For these reasons, traffic engineers put a
lot of thought and analysis into the design of intersections and how to make them as safe
and efficient as possible. Controlling the flow of traffic through an
intersection, otherwise known as assigning right-of-way is an enormous challenge and
almost always requires a compromise of numerous conflicting considerations, including space,
cost, approach speed, cycle time, sight distance, types and volumes of traffic and human factors
like habits, expectations, and reaction times. Intersections also need to be rigidly standardized
so that, when you come to an unfamiliar one, you already know your role in the careful
and chaotic dance of vehicles and pedestrians. From a throughput standpoint, the ideal intersection
would cause no interruption in flow whatsoever, but you can’t put a high-five interchange
on every city block. On the other hand, simple signs are cost-effective
and don’t require any extra space, but they can’t handle a lot of volume because they
create an interruption for every single vehicle passing through the intersection. You can see why traffic signals are so popular. They aren’t a panacea for all traffic problems,
but they do offer a very nice balance of the considerations we discussed before: Relatively
low cost, minimal space requirements, and able to handle large volumes of traffic with
only some interruption. In their simplest form, traffic signals are
a set of three lights facing each lane of an intersection. When the light is green, that lane has the
right-of-way to cross. When the light is red, they don’t. The amber light warns that the signal is about
to change from green to red. Beyond this basic function, traffic signals
can take on innumerable complexities to accommodate all kinds of situations. Let’s take a look at a typical intersection
here in the U.S. to show how they work. At each approach to the intersection, there
are three directions vehicles can go called movements: right, through, or left. Right and through are usually grouped together
as a single movement, so a typical four-way intersection has 8 vehicle and 4 pedestrian
movements. These movements can be grouped into phases
of the traffic signal. For example, the left turn movements on opposite
approaches can be grouped into a single phase because they can both go at the same time
without conflicts. Traffic engineers use a ring-and-barrier diagram
to sketch out how different phases of the signal are allowed to operate. Here’s a ring-and-barrier diagram for our
example intersection. The first phase is the major street left turns,
then the major street vehicle and pedestrian through movements, a “barrier” to clear
the intersection, the minor street left turns, the minor street vehicle and pedestrian through
movements, and finally another “barrier” before the cycle starts again. There are an endless variety of phasing arrangements
that traffic engineers use to accommodate various intersection configurations and traffic
volumes for each movement. Even the simple decision of whether to use
protected or unprotected left turns takes a significant amount of analysis and consideration. Another important decision is how long each
sequence of a phase should last. Ideally, a green light should last at least
long enough to clear the queue that built up during the red light. This isn’t always possible, especially during
peak times on busy intersections. In these cases where the intersection is saturated,
the green light might be extended for each phase to minimize the startup and clearance
times, which are periods when the intersection isn’t being utilized to its maximum capacity. The amber light needs to last long enough
for a driver to perceive the warning and decelerate their vehicle to a stop at a comfortable rate. One second for every 10 miles per hour or
16 kilometers per hour on the speed limit is a general rule of thumb, but traffic engineers
also take into account the slope of the approach and other local considerations when setting
the timing for yellow lights. In most places in North America, you are allowed
to enter an intersection for the full duration of a yellow light, which means there needs
to be a time when all phases have a red light to allow the intersection to clear. This clearance interval is usually about a
second but can be adjusted up or down based on speed limit and intersection size. So far we’ve only been talking about signals
on a set timing sequence, but most traffic signals these days are more sophisticated
than that. Actuated signal control is the term we use
for signals that can receive input from the outside and use that information to make decisions
about light timing and sequence on the fly. These types of signals rely on data from traffic
detection systems. These detectors can be video cameras or radars,
but most commonly they are inductive loop sensors embedded into the road surface. These are essentially large metal detectors
which simply measure whether or not a car or truck is present, sometimes to the annoyance
of bicycles, scooters, and motorcycles that may be too small to trigger the loop. Whatever the type of sensor, they all feed
data into an equipment cabinet located nearby. You’ve probably seen hundreds of these cabinets
without realizing their purpose. Inside this cabinet is a traffic signal controller,
essentially a simple computer that is programmed with specific logic to determine when and
how long each light will last based on the information from the detectors. Actuated control gives a traffic signal much
more flexibility to handle variations in traffic load. For example, if a nearby road is closed and
traffic rerouted through a signal that doesn’t normally see such a high demand, it may need
to be reprogrammed before the closure. A light equipped with actuated control will
simply see the additional traffic and adjust its phasing accordingly. Same thing with special events, like concerts
and sport games, that create huge traffic demands on irregular schedules, and even seasonal
changes in traffic, like in major tourist destinations. Actuated systems can also keep you from waiting
at a long light when no one’s crossing in the other direction. Finally, actuated control can help by giving
priority to emergency vehicles and public transportation by using specialized detectors,
like infrared or acoustic sensors, that communicate directly with certain types of vehicles. But, actuated control isn’t the end of the
complexity. After all, it still treats each intersection
as an isolated entity, when in reality each signal is a component of a larger traffic
network. And each component of the traffic network
can have impact, sometime a major impact, on other components in the system. Take the classic example of two signals closely
spaced in a row on a major roadway. If one signal gives a green but the next one
doesn’t, cars can back up. If they back up far enough, they can sit through
multiple cycles at an intersection without being able to pass through until the light
beyond clears. It’s a frustrating experience for anyone:
a signal is inadvertently, but significantly, reducing the capacity of an adjacent signal. One solution to this problem is signal coordination
where lights can not only consider the traffic waiting at their intersection but also the
status of nearby signals. This is a very common configuration on long
corridors with relatively minor, but frequent cross streets. The signals on the major road are timed so
that a large group of vehicles, called a platoon by traffic engineers, can make it all the
way through the corridor without interruption. This type of signal coordination can significantly
increase the volume of traffic that can pass through intersections, but it really only
works on stretches of road that don’t have a other sources of traffic interruptions like
driveways and businesses. If the platoon can’t stick together, the
benefits of coordinating signals mostly get lost. The obvious next step in efficiency is coordination
of most or all the signals within a traffic network. This is the job of adaptive signal control
technologies, or ASCT. In adaptive systems, rather than individual
groups of lights, all the information from detectors is fed into a centralized system
that can use advanced algorithms, like machine learning, to optimize traffic flow throughout
the city. These types of systems can dramatically reduce
congestion, but they’re only just starting to be implemented in major urban areas. As sensors become more ubiquitous and computing
power increases, traffic management may slowly but surely be relegated from civil engineers
to software developers and data scientists. But, that also means that ASCT systems may
be more vulnerable to security threats, a scary thought if they’re controlling the
signals for an entire city. On the complete opposite side of centralization,
many believe that self-driving cars are the next revolution in traffic management. If every vehicle could communicate and coordinate
with every other vehicle on the road, interrupted traffic control could eventually become a
thing of the past. But don’t get your hopes too high. In dense urban areas, traffic congestion is
often self-limiting. Especially during peak times, for every one
person on the road, there are many more at work or at home waiting for the congestion
to clear up before they head out. This latent demand means that any increase
in capacity will quickly be filled up with more traffic, bringing the congestion back
to the same level it was before. However we accommodate it now or in future,
traffic will continue to be one of the biggest challenges in our urban areas and traffic
signals will continue to be one of its solutions. Thanks to Squarespace for sponsoring Practical
Engineering. In a previous video, I asked you for suggestions
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Just occured to me that "Grady" is a portmanteau of Grey and Brady ...
Coincidence?? I think not