Tracking the trail: Sensors on Mount Vernon path collect data to aid transportation planning

September 2, 2013

John Pickett, a federal economist, mounts the 20-year-old red touring bike that he’s dubbed “the Mule” just before 7 a.m. outside the shed behind his home in the Mount Vernon area of Fairfax County. Over the past decades, he’s put more than 33,000 miles on its odometer, and he’s ready to add 15 more.

As the rising sun breaks over the Potomac River, Pickett pedals northward along the Mount Vernon Trail. He is one of the Washington area’s growing number of bicycle commuters, and he’s due at work in Rosslyn in about an hour.

Getting into his rhythm near Dyke Marsh, the 57-year-old Pickett barely notices passing a brown wooden post. He has no sense of breaking a light beam, but this is where the National Park Service first detects — and counts — him.

It happens over and over again to Pickett and the approximately 2,000 others who use the trail each day. As they bike, run and walk the 18.1-mile path, they unknowingly trip a series of sensors. Those dozen sensors are connected to counters, which over time will tell a tale of who is using the route and at what times.

In the traffic-snarled capital region, where transportation alternatives are becoming increasingly important, this bike and pedestrian route and others like it across Northern Virginia, Maryland and the District have become “a secondary nervous system” for commuters.


“We know a lot about cars,” said Ralph Buehler, an associate professor of urban affairs and planning for Virginia Tech who has consulted with the Park Service. “Suddenly, we have data that enlarges what we can see about how bicyclists and pedestrians behave.”

Planners now know that runners and bikers will be out in the cold, heat and snow but that when it rains, trail usage drops dramatically. During the workweek, the northbound commute starts before 5 a.m. and tapers off by 9 a.m. An equivalent southbound traffic bulge runs from about 4 until 7 p.m. Weekends are a different story; while bicyclists still outnumber runners and walkers, the foot traffic grows and the commuting pattern disappears, replaced by a long midday bulge.

But the raw data from the dozen counters along the trail need to be properly examined, obvious errors need to be accounted for and trends identified. Unlike local or state government transportation departments, which staff analysts who look at the area’s road traffic, the Park Service has no such expert for non-motorized travel.

“We have a lot of data but not enough capacity to do the analysis,” said Thomas Sheffer, a planner with the Park Service’s George Washington Memorial Parkway. “But I think what we’re seeing is a reflection of national trends.”

The technology

Bike-friendly Arlington County, which works closely with the National Park Service, has placed the newest sensor, an expensive, state-of-the-art, French-made device, on the trail near Reagan National Airport.

Unlike older counters, this one can distinguish between humans and machines, using an induction loop embedded in a double-diamond pattern in the pavement. It senses when a bicycle passes over. That is paired with a passive infrared sensor that notices the direction of travel when a warm body passes by, creating a disturbance much like a boat’s bow wave. That minute-by-minute information is wirelessly uploaded each quarter-hour to a central database and sent to the county daily.

Arlington is part of a research project sponsored by the National Academy of Sciences in bike meccas such as Portland, Ore.; Minneapolis; San Francisco; Davis, Calif.; and Montreal to figure out how best to capture bike and pedestrian usage data on urban trails.

“I’m swimming in data,” says David Patton, Arlington’s bicycle and pedestrian planner, as he monitors streams of numbers.

Arlington has 29 sensors it uses for all kinds of planning purposes, including for sidewalk width and crosswalk installation. With another four or five on order, it has the region’s most extensive bike and pedestrian tracking system. The Park Service has been counting recreational users on this particular trail since 1988, at first with volunteers with clipboards and later with a variety of automated sensors.

While pedestrians and bicyclists share the trail, reports show that two-thirds of the users are on two wheels. No one knows how many are commuter, but bright red Capital Bikeshare bikes are popping up with regularity.

That rent-by-the-hour bike program started with 100 stations in the District and 14 in Arlington in 2010. It now has more shared bikes in circulation — 1,890 — than any other region in the country, having just flown by the 4 million trip mark, with 22,000 members and expanded networks in the District, Arlington, Alexandria and, soon, Montgomery County. Bike commuting in Arlington has more than doubled between 2006 and 2011, and the D.C. Department of Transportation estimates an average of 87,000 bike trips per day in the region.

The commute

Pickett figures he logged 585 commuting miles on his bike in July alone. “That’s about 22 gallons of gas or $55-plus in my pocket,” he wrote on his blog, A Few Spokes Shy of a Wheel.

For well over 10 years, Pickett has biked the Mount Vernon Trail’s winding, slightly inclined route, coping with limited lines of sight, places where headlights blind bikers, and the ever-present tourists, bird-watchers and strollers who wander across the four-foot-wide trail. He has learned to keep an eye out for “ninjas,” people clad entirely in black who run before sunrise or after sunset without lights or reflectors.

He and others also report what the data show — the number of riders has spiked upward in the past few years.

“There have been times on the trail when I felt like I was in a car on the highway, with bicycles as far as the eye can see,” Pickett says.

The counters that Pickett passes at his 12-mph clip should have no problem recording his daily passages. But the unpredictable happens. Someone spots the nice wood post along a popular trail and slaps a lost-dog flier on it, blocking the electronic beam. Spiders spin dense cobwebs across reflectors, which can stop an electronic signal. Runners use posts to stretch or landscapers bump into them, jarring the beams ever so slightly out of alignment. Deer and raccoons meander around them.

“Is it a flock of Canada geese? Is it the Marine Corps Marathon?” Arlington’s Patton asks.

Neither. It’s an invasion of ants, flooded out of their nests during a particularly rainy month. The insects find a warm, dry spot inside the posts that house the infrared sensors. All those little legs scurrying across the light beam were not recorded as runners, but their movements were enough to confuse the electromagnetic counter. Patton heads to the site with a shovel and a container of boric acid to discourage the critters from returning.

Bob Clark, 75, a retired program analyst with the Department of Health and Human Services who volunteers for the Mount Vernon Trail patrol, manages the Park Service database where the trail usage numbers are stored. He collects the data during his frequent foot patrols along the trail, downloading the information with a key fob or a small device called a “DataHog.” He is careful not to overstate the value of numbers he has been collecting, although he hopes the record that is being established will prove useful. Twice on one recent Friday morning, he was sought out for help by trail users.

After solving both problems, he dryly noted, “Statistically speaking, this is outside the norm.”

The future

The Park Service, which relies on volunteers such as Clark, hopes to eventually find a graduate student who can analyze the decades of data that will help them manage the trail. Patton, who has more than the Mount Vernon Trail to track, says he is satisfied to collect the data for now and do the analysis later. In the meantime, walkers, runners and bicyclists stream up and down the trail.

Pickett passes by the airport, past the drivers waiting to get on Interstate 395 and under the Memorial Bridge, where the trail narrows to one lane. He rides the boardwalk beneath the Theodore Roosevelt Bridge, which he says “can be very treacherous.” He snakes around the parking lot at Roosevelt Island and then powers uphill to North Lynn Street, where the trail ends.

But his morning commute is not quite finished.

“The I-66 off-ramp comes to a red light right next to the bike trail,” Pickett says, providing the kind of input no sensors can match. “Almost every single day, people blow that light to take a right on red. There’s a bad sightline for drivers to the left, and I’m coming out of a spot most people don’t expect. But it would help if the drivers would actually stop there.”

He negotiates the I-66 ramp crossings, weaves through a crowd of workers headed to the high-rises of Rosslyn and enters the garage to his own office at mid-block on Lynn Street. Inside, he locks his bike to a rack with six others, including one that belongs to his boss, who rides into Arlington from Petworth. He comes via another set of trails and bike paths.

Patricia Sullivan seeks out news about Alexandria and Arlington County for the Washington Post.
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