Our annual ranking of America's top private fleets has reached a milestone with this fifth edition. For the first time, the companies included in the Fleet Owner 500 operate more than 1-million trucks and tractors. To put that into perspective, there are 8.1-million commercial vehicles in the U.S., which means the largest 500 private fleets account for more than 12% of all the trucks on our roads.

The Fleet Owner 500 was created five years ago to recognize the size and scope of private fleet operations. Often called the invisible side of the trucking industry, these fleets provide indispensable support to all types of businesses that don't generally think of themselves as being in trucking. However, without their trucks, these businesses wouldn't be able to distribute products, deliver services or keep supply chains moving — in short, succeed in their primary missions.

Rankings are based on total power units controlled by a fleet, and we also provide a breakdown of tractors and trailers for each fleet, as well as trailers and the parent company's business category. The data is collected by Fleet Owner's FleetSeek using a number of public and proprietary sources that are verified by follow-up telephone interviews with fleets.

Since they rarely operate as separate business entities, private fleets can be elusive, so each year we make a special effort to expand our search to bring in the broadest number of potential candidates for our Top 500. This year the list includes 145 new companies not found on the 2007 edition. In some cases, we rolled up numbers from a few subsidiaries into a single fleet number under the parent company's name, but many are new companies found through efforts to reach more of the electric utility industry.

In general, the fleets at the top of the list remain unchanged, although combining AT&T subsidiaries pushed that fleet from No. 2 in 2007 to No. 1 this year. At the other end of the list, the cut-off point for making the Top 500 jumped from by almost 40 power units, eliminating many smaller fleets identified in last year's list.

Overall, the total number of power units controlled by the Top 500 grew 18.9% compared to last year. All growth was in trucks (up 22.9%), as the tractor total actually saw a slight 1.9% decline. The trailer pool also expanded this year, showing an 11.9% jump.

Following the Top 500 rankings, you'll find a breakdown on vehicles operated by business category, as well as the top five fleets in each of those categories and the top ten fleets by tractor totals and by truck totals.