All hands on deck: Traffic speed study will reveal a lot

| 5/21/2008

Picturing himself as a handyman isn’t easy, admits Steven Johnson, but he made a promise to

himself to build a deck this summer at his home in Fayetteville, AR.

A professor of industrial engineering at the University of Arkansas, Johnson spends his usual working hours being one of the most highly sought-after traffic gurus in the transportation industry. His data are invaluable, particularly in trucking, according to those who swear by his work.

Steven Johnson
(Photo courtesy of the University of Arkansas)

With his major study on traffic speed and speed differentials wrapped up and about be published by the American Transportation Research Institute, Johnson decided to treat himself by purchasing tools needed to build his deck.

He affably admits he’s no Bob Vila. During a telephone interview May 13 with Land Line, Johnson quipped that his wife is keeping an eye on him in case he hits his thumb with a hammer.

A week ago Johnson was up to his eyeballs in traffic data, averages, graphs and charts as he prepared his findings on posted speed limits and speed differentials between trucks and passenger vehicles. He knows the fast lane because he lives in it most of the time.

Johnson’s latest offering is set to show that speed differentials between trucks and automobiles exist no matter what the posted speed limits are.

“What it amounts to is that the posted speed limit for cars in particular is irrelevant,” Johnson said. “The minimum differential is about five and a half miles an hour, even in states with uniform speed limits.

“The average speed (for cars) is about 72 to 73 miles per hour across the country whether it’s a 65 mph speed zone or a 75 mph speed zone.”

The large majority of trucks, on the other hand, had a tendency to travel at speeds at or below the speed limit, rarely topping 70 mph, even when the speed limit was 75 mph.

“And that’s primarily because of the limiters on the fleet trucks and the limiters that the owner-operators use as well,” Johnson said.

Johnson’s data are expected to start people thinking because the number of fleets rolling back their speed limiters to 65 mph, or 62 mph, is increasing because of higher diesel prices.

“They’re expecting to have a better return on their investment than they’re going to get, from my standpoint,” he said.

The Technology & Maintenance Council estimates that rolling back the speed limiter on a truck equates to a savings of 0.1 mile per gallon for each mph decreased.

Johnson is always ready to challenge those numbers, which he says come from old data.

“We really think that depending on how much idling you have, it could be more like 0.03 – or somewhere between 0.03 and 0.05.”

Johnson set out years ago to find the answer to the question: “What is actually going on out there on the highways?” Posted speed limits, flow of traffic and vehicle class have always fascinated him.

ATRI, the research arm of the American Trucking Association, has published several of Johnson’s studies.

His findings don’t always show what the major fleets want or expect.

Johnson is not funded by motor carriers, trucking companies or associations, and that is why his data are held in high regard, according to Tom Weakley, director of the OOIDA Foundation at the Missouri-based Owner-Operator Independent Drivers Association.

“The thing I appreciate about Dr. Johnson’s study is that he hasn’t been influenced by those things,” Weakley told “Land Line Now” on XM Satellite Radio.

Johnson is the first to admit that speed and fuel consumption are closely related, but he also points out that vehicles interacting with slower trucks will burn more fuel.

“My main contention is the loss of fuel from the deceleration and acceleration,” Johnson said. “I still contend that the slowest truck on the road is going to save fuel, no doubt in my mind – but in the process costing all of the other trucks and the rest of the citizenry an amazing amount of fuel and emissions because of the deceleration and acceleration, which suck it up like mad.”

Another interesting aspect to Johnson’s study data is his thorough debunking of the Federal Motor Carrier Safety Administration’s “Large Truck Crash Causation Study.”

Johnson said the causation study does little to show what actually causes crashes, and it refrains from an important distinction. The study does not separate “speeding,” defined as traveling over the posted speed limit, from driving “too fast for conditions.”

“In the study, for crashes on rural interstate accidents that occurred, it turns out that not one of them – and there were 109 on rural interstates – was going above the posted speed limit,” Johnson said.

– By David Tanner, staff writer