The thump, thump is barely noticeable to those behind the wheel. State traffic survey engineer, David White, takes note of every thump.
"The air pulse activates basically a microphone switch," White explains. "You have an electronic timer and memory in there that records the information."
Over 1,000 such boxes fill the Raleigh area. Crews collect the data daily to create new vehicle count maps. On Battle Bridge Road, 1,200 cars per day were recorded in 1995.
State highway planners use the maps to determine which roads need improvements like widening, signal lights or even mass transit. The greatest need for the information actually comes from a demand by the public.
For a small fee, anyone can buy the vehicle count maps to decide where to build new homes, new stores or restaurants. If roads like one section of Rock Quarry keep up with the state average, then almost 200 more cars a day will hit the counters during this survey than was the case five years ago-- maybe more, maybe less.
Similar vehicle counts led to the current widening of Interstate 40 between Raleigh and the Research Triangle Park.