The number of traffic tickets issued by Philadelphia police has dropped 35 percent over the last five years, prompting the department to "reemphasize" that officers are supposed to write violations when they see them.
"I don't think driving habits have improved," said police spokesman Lt. Ray Evers.
Tickets for running a red light are down 43 percent since 2007, while citations for driving without insurance are down 49 percent.
At the same time, the number of cars registered in Philadelphia has, on average, remained stable.
Police call tickets for moving violation "movers," and Evers agreed that "movers are down."
"There are a myriad of different reasons why that is happening," he said, including an intensified focus on violent crime - meaning less emphasis on traffic.
Nevertheless, "part of our job is enforcement," he said. "It's important that our officers are writing violations when we see them."
While stops by police officers are down, red-light-camera citations have increased as more cameras are installed.
Evers said there had been an uptick in moving-violation citations in the last two months, but he could not immediately provide data. "We reemphasized that this is what we should be doing," he said.
Officers are told that, in the course of regular patrols, they need to keep a sharper watch for traffic violations.
The data on enforcement were generated by the Philadelphia Traffic Court.
The sharp drop in tickets also coincides with higher gasoline prices and a dismal economy, all of which could also be a contributing factor, though ones that are not easy to measure.
Then there is technology, old and new.
"There are more red-light cameras," said Jenny Robinson, public affairs manager at AAA Mid-Atlantic, who wondered whether red-light enforcement was being supplanted by the cameras.
In fact, red-light cameras have proliferated in Philadelphia. In the fiscal year ended March 31, 85 cameras at 19 intersections took 141,571 automated photographs of vehicles running red lights.
And as the price of gasoline has risen, people are driving less, Robinson said. SEPTA and PATCO ridership is up, she said, and the traffic over the Delaware River bridges "is at an 11-year low." And fewer workers mean less rush-hour traffic and less chance of an accident.
The large volume of traffic data produced by the state and city add other elements to a complex picture of the highway transportation scene.
Despite the drop in enforcement, the number of traffic accidents in Philadelphia has dropped in absolute and relative terms between 2006 and 2010, from 11,682 to 10,965. One factor could be a drop-off in traffic.
There have also been slight drops in each of the suburban Pennsylvania counties, with the exception of Montgomery, where accidents plunged from 9,788 to 8,284, or 15 percent. Statewide, the drop was 5.5 percent.
Technology is probably a factor, according to research by the Insurance Institute for Highway Safety.
Antilock brakes, which help prevent skids in high-speed stops on dry roads and all stops on wet roads, are now in almost every car on the road. In 2006, the figure was still about 90 percent.
Electronic stability control, which uses the brakes to compensate for a loss of traction at high speed, was available on only about half the cars produced in 2006. It is now standard equipment on more than 80 percent of vehicles.
There is no single cause, but "crashes have been on a downward trajectory for about a decade now," said Russ Rader, vice president for communications at the insurance institute.
Like the AAA, he said that "a big factor is likely the recession."
"The silver lining for any economic downturn is fewer crashes because there is less driving," Rader said, but he added that a variety of other factors also contributed.
"Running red lights, or traffic signals, is the number-one cause of crashes in urban settings, so the red-light cameras are likely a factor," Rader said.
But in Philadelphia last year, police data showed accidents ticking up at intersections with red-light cameras. That has been disputed by the Philadelphia Parking Authority, which runs the system.
A red-light-camera ticket produces no points, but it has generated $13.7 million in revenue.
After $6 million in expenses, the leftover $7.7 million was turned over to the state, which then gives half of it back to the city.
Rader also said red-light-camera programs produced "a well-known spillover effect: Drivers are not always aware of exactly where the cameras are, but know it is in effect."