By David Tanner
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.
A professor of industrial engineering at the University of Arkansas, Johnson spends his usual working hours being one of the most highly sought-after gurus in the transportation industry. His data are invaluable, particularly in trucking, according to those who swear by his work.
With his latest study on traffic speed and speed differentials complete and about to be published, Johnson decided to treat himself by purchasing tools needed to build his deck. He affably admits he’s no Bob Vila. During a recent telephone interview with Land Line, Johnson quipped that his wife was keeping an eye on him in case he hit his thumb with a hammer.
A week earlier, Johnson had been 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 data show that speed differentials between trucks and passenger vehicles 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 mph 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 posted limits, rarely topping 70 mph, even when the speed limit was 75 mph, according to Johnson’s data.
“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 findings started out five years ago as a data collection funded by a U.S. Department of Transportation grant to the Mack-Blackwell Transportation Center at the University of Arkansas. They have become hot property throughout the trucking industry.
The American Transportation Research Institute is currently using Johnson’s data on speed differentials and posted speed limits for an upcoming report. Johnson also contributed fleet data to ATRI for that study.
While collecting and analyzing data for various speed studies, Johnson has made independent findings about fuel consumption and other issues.
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 said those numbers were derived from old data. He said he believes a realistic savings would amount to about half of the TMC figure.
Johnson said he 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 used several of Johnson’s studies. His findings don’t always show what the major fleets want or expect to see.
Johnson is quick to agree 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 cost all of the other trucks and the rest of the citizenry an amazing amount of fuel and emissions because of the deceleration and acceleration.”
Johnson uses data from the Federal Motor Carrier Safety Administration’s “Large Truck Crash Causation Study” as part of his own studies.
He pointed out that the causation study does little to show what actually causes crashes, and it fails to 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 …, it turns out that not one of them – and there were 109 on rural interstates – was documented as going above the posted speed limit,” Johnson said.
What lies ahead for the man in the fast lane?
Johnson said he is collecting data that pay special attention to the effects of speed differentials on interstate congestion. LL