Rescue Vehicles

Throughout Left 4 Dead, there are four rescue vehicles, each on the fifth and final chapter of any campaign. The goal of the Survivors is to reach these vehicles, which will take them to a point that is safe from The Infection. Each vehicle is summoned via some type of radio, and each vehicle takes approximately fifteen minutes to arrive at the Survivors' position. In the mean time, the Survivors are kept busy with multiple waves of The Infected.

Rooftop Finale
Through the city of Fairfield, a helicopter called News Chopper 5 flies through the skies, calling out to any Survivors left in the city. The pilot tells anybody who can hear him to get to Mercy Hospital, where he can pick them up. At the end of the No Mercy campaign, the Survivors reach the roof of Mercy Hospital and find a radio to summon their rescue with.

Boathouse Finale
Outside of the small town Riverside is a boathouse. On a table inside is a radio, being hailed by John and Amanda Slater, who own the S.S. Saint Lidia II. They have been circling their boat around the river, calling the radio in hopes of finding people who have survived the Infection. Any survivors would be taken to an army safe point up the river. At the end of the Death Toll campaign, the Survivors find the boathouse and send for their rescue.

Runway Finale
At the beginning of the Dead Air campaign, a military plane flies over the city of Newburg, heading to the Metro International Airport. It finds the airport has been obliterated under the Infection, and any personnel that would otherwise be able to help the pilot has been Infected. However, the Survivors reach the airport at the end of the campaign, and answer a radio which is in the hands of a corpse. They start the gas pump, and wait through hordes of the Infected for the tank to fill up.

Farmhouse Finale
In a rural area, the army had locked down a farm to help Survivors escape the Infection. At least, until they themselves were forced to leave. However, they did not give up all hope on anybody surviving; they left a radio behind. At the end of the Blood Harvest campaign, the Survivors reach the farmhouse where the radio is, and call for their rescue, which arrives in the form of an APC vehicle.

Trivia

 * Originally, Valve had the idea of News Chopper 5's pilot saying he wasn't feeling too well after his last trip to rescue some survivors. Shortly after this, the pilot would die, and the helicopter would crash, beginning the Death Toll campaign. However, they felt that this would be an unsatisfactory ending to the campaign, and cut it out.