Testing pavement friction in 1977

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april 1977

By Angela DeWelles / ADOT Communications

This photo appeared in the
April 1977 issue of NewsBeat, a
publication that served up agency
news to ADOT employees for decades.
Printed alongside of it was this caption:

John Burns of Materials Services prepares
to shoot a scene with a Super-8 mm
camera for a 25-minute sound film he’s
producing on how differential pavement
friction affects skidding vehicles. Burns
was a chief investigator on research.
FHWA (the Federal Highway Administration) is funding the movie to be shown in
other states.

That’s not a whole lot of information
to work with, and we have so many
unanswered questions. Good thing David
Allocco, a.k.a. the guy in the driver’s seat,
agreed to share some additional details.

Allocco, who started with ADOT in March
1973, works today in the Bridge Group’s
Geotechnical Section as a
materials source engineer.
While his job no longer
involves measuring pavement
surface friction, he distinctly
remembers driving that

Used to test the wet friction
characteristics of a roadway,
the car had special features,
including special bald
skid-testing tires, a roll bar
and a harness. The safety
equipment was necessary
because of the testing

“We’d have a water truck that
would wet down the road,”
Allocco said. “Then, I’d get
the car up to speed, put it in
neutral and then you’d have to
slam on the brakes as hard as
you could with both feet.”

Engineers would measure
how far the car skidded and would use
the distance to determine a coefficient of
friction, Allocco said.

While the vehicle was typically used for
research and to test out small sections of
various roadways (another device known
as a Mu-meter was used to test longer
stretches of pavement), Allocco remembers
a unique request that came from the Tucson
International Airport.

“The Air Force had been doing touch-and-go
landing exercises at the airport, and
they wanted us to test to see if the friction
was affected,” said Allocco, adding that
he skidded 655 feet and ended up farther
than expected, past the wet part of the

“That was the most exciting time I ever
drove it,” he said.

Today, ADOT still tests pavement friction,
but the methods have evolved.

Now something called a Dynatest Highway
Friction Tester is used, said Pavement
Condition and Evaluation Manager Kevin
Robertson. It’s a truck with a 300-gallon
water tank and an extra wheel that, when
a test is activated, mimics what vehicles
experience when braking on the wet road.

“It tells us the macrotexture of the
road,” said Robertson, adding that the
sophisticated onboard technology
calculates a friction number ranging from
0 to 100, giving engineers the data they
need to make decisions about pavement

“When it comes to the ability to test
pavement friction, we have the most
advanced and safest machine available to
do that,” Robertson said.

It’s safe to say things have changed since 1912 when the Arizona Highway Department was first established. But you don’t just have to take our word … we’ve got plenty of pictures to prove it. We combed through our archives and decided to periodically post these photos from the past in a blog series we’re calling, “From the ADOT Archives.” 

Source: Arizona DMV

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