Traffic counters are often subtle and hard to spot. But “why here?” is the bigger question.

After over 800 articles penned, I have consistently asserted that the point of this blog is to put forward empirical arguments.  I try to extract greater meaning from what I observe.  Data and second-party research supplements or substantiates the arguments I try to make.  Not every blog post uses outside articles, legal documentation, and/or statistics.  But they all have at least one photo.  (Well, almost every article.)  And, more often than not, I don’t go out with any intention of “meeting the subject” of my articles; they arrive at me completely unexpectedly.  Given that these unexpected encounters often occur while I’m in a car, I’m not always behaving safely, snapping photos when stopped at a light.  But if I see something interesting and I can whip out a camera phone in time, I still do it.  And, in recent years, I have gotten in the habit of running with my smart phone, primarily to track pace and distance using one of the many apps available.  But it has the added benefit of allowing me to pull that camera out if I see something interesting (like a traffic counter) during the run. And that’s exactly what happened for this article.  Twice.  Two different runs.  Two different traffic counters.

The first traffic counter encounter took place a few weeks ago when stopped at Richmond Highway (US 1), a busy arterial that bisects Alexandria, Virginia.

This is the US 1 intersection with East Glebe Road, a collector that generates its own fair share of traffic.  It’s an intersection I’ve crossed numerous times over the last few years; I’d still say I pass it at least three or four times per week, usually with four wheels.  Thankfully, this time, I was behaving myself: I wasn’t in my car but instead was jogging on the sidewalk—a whole lot safer under those conditions to whip out a camera phone.

Regardless of where I stood when I took the photo, the image clearly features something out of the ordinary, which, since I’ve framed the photo pretty effectively (and have already given it away with the title), should be obvious.  Those orange bands about two feet up from the pedestrian crossing pole tether a similarly orange vise, which stabilizes a stanchion projecting upwards out of the photo’s frame.  It’s not elegant: some of the orange stabilizers look more like they’re just ties to help keep the stanchion perfectly straight; or even maybe something akin to electrical tape.  But they’re doing their job.

The stabilizing forces look pretty strong, hopefully tamper-proof among pedestrians in the area.  (At the very least, it would take long enough to effectively dismantle the stanchion that someone going past this busy street would notice that something is amiss…and hopefully report it.)  

But what is the device at the top of this pole?  Of course; it’s one of the aforementioned traffic counters: a tool for determining traffic volume at this particular intersection of Richmond and East Glebe.  But traffic counting devices can come in a variety of configurations and means of technological delivery.  Time to glance upward and see what’s at the top of that big pole. 

VID traffic counters in Alexandria, VA

It looks like a smaller version of the pedestrian crossing signal, whose illuminated “WALK/DON’T WALK” side points into the crosswalk—the opposite side of where I stood for this photo.  But just above the signal is one of those traffic counters: a little box above the bigger box, with what appears to be a suitcase-style handle protruding from the left side.

Now, if only I could figure out what type it is or how it works.  A variety of technologies are available to transportation engineers and planners to achieve their goals, depending largely on the resources (money) available and the desired result.  I’m not an expert by any means in identifying the types, but my guess is this is one of two traffic counters: either radar or video image detection (VID).  Both are a bit on the pricey side, but they are good at not just taking a count but in allowing discernment of different types of vehicles: passenger cars, freight carrying tractor trailers, construction vehicles, etc.  They are also optimal at distinguishing if motorists are moving at slower speeds than usual due to congestion, and when those periods of congestion seriously compromise the Level of Service (LOS) of a certain road segment or intersection.  But these two high-tech traffic counters have their drawbacks, well articulated in the website I linked earlier.  Radar traffic counters, which I’d estimate have been around longer, are prone to mistakes during periods of high congestion.  Meanwhile, the VID traffic counters allow machinery to “see” the road—using emergent technology similar to the safety measures that may eventually elevate self-driving cars to the status quo.  But VID is inevitably compromised by the same conditions that compromise human vision: poor weather and nighttime darkness.  They also can prompt privacy issues akin to the devices that Google Street View (and, thus, this blog) have depended upon for years.

The traffic counters here seem to point directly onto East Glebe Road (collector) rather than Richmond Highway (an arterial).  The opposite corner has one hoisted into the air the exact same manner:

VID traffic counters in Alexandria, VA

Due to their position, I’m going to wager that these are VID-based traffic counters.  If they were beam-shooting radar counters, it would be more advantageous for them to shoot laterally at ground level, rather than an awkward acute angle hoisted 20 feet in the air.  Conversely, that elevation confers advantages to “seeing” traffic counters like VID, even if the technology is still comparatively new and thus more expensive.  As for the location, these traffic counters scope an area in the midst of significant change: East Glebe Road terminates a few blocks to the east, right at the Potomac Yard Metro Station, which opened less than a year ago.  With a brand-new WMATA metro stop in the area, already high land values are surging even more.  Back in the summer 2020, I noted how a huge Regal Cinemas multiplex closed after COVID lockdowns, but it wasn’t due to the lockdowns themselves, as tough as that was for the theatre industry.  Virginia Tech University purchased the land, demolished the theater in 2021, and built a new extension campus.  The combination of this fancy new use and the metro stop have undoubtedly prompted a reassessment of traffic flows where East Glebe meets Richmond Highway.

My article on traffic counters would have ended with this simple observation, but—lo and behold!—it gets delayed from the pre-planned delivery date of February 29th.  No Leap Day post, because I stumbled across yet another slew of traffic counters during a run, this time in neighboring Arlington County.

Doesn’t exactly look the same, does it?  Where are the traffic counters?  It’s those three narrow black lines stretched across the road.  It’s easier to see in its full context.

pneumatic tube traffic counters in Arlington VA

Another little box for capturing and processing the information.  It even has a similar suitcase handle on the side, which, alas, is barely visible due to the shadow.  But it’s easy to infer where the handle would be.

I’m going to go out on a limb and wager that these are pneumatic tube traffic counters, stretched across a street perpendicular to traffic flow, registering a new count to the box each time a car drives over the tubes.  They’re a time-tested tool, much cheaper than radar or VID.  They can detect axles, which allows them to distinguish big rigs or buses from single-passenger cars. 

pneumatic tube traffic counters in Arlington VA

Several of the pneumatic tubes stretch across a relatively minor local road: South Lang Street, a two-block road that provides a shortcut from Arlington Ridge Road to a shopping plaza that would be visible on the righthand edge of the photo below.

Even though these traffic counters are less than a mile away from the Richmond/Glebe example I just featured, the purpose is almost certainly completely different.  And that different purpose no doubt helps explain the choice of pneumatic tubes.  If all that’s necessary is a raw number collected over a certain time interval, pneumatic tubes are probably the best bet: many planning departments already own them.  (I worked for one in the past.)  Beyond just the lower cost, pneumatic tubes are reliable at getting those numbers and, if deployed over certain preset intervals across multiple days, can effectively determine peak times for traffic or if surges take place certain days of the week.  Conversely, they can’t offer much nuance for different vehicle classes or determine how fast they were going.  And speed of vehicles is often critical in accurately estimating a road segment’s LOS.

pneumatic tube traffic counters in Arlington VA

South Lang Street is a local road abutting single-family homes and a school, yet it’s also a popular recommendation among GPS apps as a cut-through to a very busy neighborhood shopping center.  Bearing this in mind, I’d guess these traffic counters are building a case for traffic calming devices (speed bumps are hugely popular in the DC metro) or even an imposed one-way on South Lang, to discourage vehicles from barreling downhill from a collector like Arlington Ridge Road onto this tiny street, moving at dangerously high speeds amidst kids walking home from Gunston Middle School.

Doesn’t really matter why these traffic counters are stretched or perched.  But then, guessing these type of things are the primary reason I keep at this blog.  Pretty impressive to take all these traffic-based pics and never have to roll down the window of my car.  Here’s hoping I can keep running for many years to come.

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5 thoughts on “Traffic counters are often subtle and hard to spot. But “why here?” is the bigger question.

  1. Jeffrey Jakucyk

    “Conversely, they can’t offer much nuance for different vehicle classes or determine how fast they were going. And speed of vehicles is often critical in accurately estimating a road segment’s LOS.”

    Actually with multiple tubes you can determine direction, speed, and vehicle classification with some data crunching afterwards. For instance, two tubes will let you determine direction based on two impact/actuation events in close succession and which came first. Two tubes a known distance apart allows you to also calculate speed based on the time between those two events. A known series of actuations that corresponds to typical distances between axles also allows classification. That said, the error rate goes up in high-congestion environments where two passenger cars crossing the tubes in adjacent lanes at nearly the same time can register as an 18-wheeler or a cement truck.

    Some ways to improve accuracy is more tubes. I’ve seen 4-tube setups where two of them go halfway across the street and the other two go all the way across. That allows the data from the shorter tubes to be subtracted out of the longer ones, and you get independent speed and classification data for each direction. I suspect the 3-tube setup may be a way to get the same data as a 4-tube just with a little more data post-processing. The fewer tubes the less time it takes to install, which is often quite dangerous.

    Reply
    1. AmericanDirt Post author

      I don’t doubt you on this, Jeffrey. Makes sense to me. In hindsight, I cited most of my research from a company that probably has a vested interest in presenting its offerings (VID and radar) as more competitive alternatives to pneumatic tubes. If there are multiple tubes in a testing mechanism and a willingness to crunch some highly precise number-crunching (and someone willing to do it), I can imagine the they will tell any information needed about axles. Easy to determine which direction they’re going, trickier to determine (but not impossible) to determine how fast they’re going, trickier still to determine if they simply driving slowly or due to some external condition, and virtually impossible to distinguish between cars with fairly similar chassises, like a Nissan Sentra from a Toyota Camry.

      In the example I cited (Lang Road), it’s pretty certain that traffic is going to be thicker going one direction than the other, since this little road affords greater advantage as a cut-through to the shopping center (and to bypass a stop light, as well as some temporary construction)–while fewer people are going to consider the road an advantage when leaving the center. That said, Lang Road already gets the speed bump treatment, so I’m not sure what traffic counts will achieve if the goal is a justification for calming. That said, I would need you to clarify how installing 4 tubes versus 2 would be more dangerous; wouldn’t the traffic modelers have the right to temporarily close the road for installation regardless?

      Reply
      1. Jeffrey Jakucyk

        My understanding is that any time a worker is in the road, even if it’s ostensibly closed, it puts them at a greatly elevated risk. They try to do installs at night when traffic volumes are less, but the darkness introduces its own dangers. I think what they often do is a single-lane closure, where they have marshals allowing alternating platoons of traffic through the unblocked portion of the road. That still puts the workers in close proximity to moving traffic, and more tubes means more time drilling, bolting, taping, and calibrating, so there’s more time exposed to traffic.

        Reply
        1. AmericanDirt Post author

          Guess that makes sense, when it’s necessary to assess safety measures against the desire for accuracy or a robust array of analytics, which is going to happen with multiple pneumatic tubes. They seem like crude, old technology, so I figured they’re comparatively easy to install, but I suppose that if they get loosened from all those cars driving over them, that will compromise the data. So there’s more to it.

          I’ve never figured out why that two-block Lang Street needs more speed/traffic measurements. It already has speed bumps. But, as I indicated, it’s already a popular cut-through, especially cars careening southbound down a pretty respectable hill from Arlington Ridge Road. And there’s a middle school immediately out of the frome of the photos, with huge percentages of kids no doubt walking home; it’s a pretty pedestrian friendly area. I unfortunately can’t post images on the comment section of WordPress, but here’s a Google Street View link that shows the full street looking southbound from the top of the hill: https://maps.app.goo.gl/sK5Lkk9BsjSGUogt8

          Chances are, any traffic analysis at this point will determine if even more traffic calming and safety features are necessary. It wouldn’t surprise me if Arlington County Public Works decides to install a small rotary at the one four-way intersection, ped crossing speed tables, or the rare (in the US) chicanes.

          Reply
          1. Jeffrey Jakucyk

            Some counties or municipalities do routine traffic counts just as a matter of course, not necessarily with any plan for upgrades or traffic calming measures. It’s just good data to have so when there is a plan that comes up, they don’t have to scramble to measure traffic at every surrounding street all at once. It may only be every 10 years for a particular stretch of road.

            Reply

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