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 ten to 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 Bell 205 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. During their wait for the helicopter, the pilot mentions that this will probably be his last trip--a remnant of another ending to the campaign.

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, which is a small fishing vehicle that has been circling around the river while John attempts to hail anybody who had survived the Infection. He and Amanda are offering to take any armed survivors to a military hold out up the river. Fortunately, there are plenty of weapons to supply people with. At the end of the Death Toll campaign, the Survivors find the boathouse and answer the radio, making use of the various firearms around the building to help them wait through the hordes.

Runway Finale
At the beginning of the Dead Air campaign, a military C-130 Hercules 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 killed or Infected. However, the Survivors reach the airport at the end of the campaign to find the plane. They answer a radio which is in the hands of a corpse next to a gas pump and agree to help the pilot, as they are otherwise stranded. They start the pump, and wait through waves of the Infected for the tank to fill.

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 asking for a first aid kit, saying that he wasn't feeling well after he made an attempt to rescue some Survivors from the street, which he knew was a dangerous move. Shortly thereafter, the pilot would die, more than likely from wounds caused by the Infected, 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, though they leave the preceding dialogue in the final version.
 * There is a large poster advertising the News Chopper 5 at the start of The Subway. It can be seen as the Survivors go down the stairs and into the first station's ticket booth area.