Are the characters in Left For Dead 1 and 2 modeled after real-life people?
It depends on what you mean by "modeled after real-life people".
If you mean visually, then yes! Valve usually models the face of their characters based on some real-life person (probably a hired model or actor, but I can't confirm this). From Left4Dead Wikia, here is a list of the face models for each character (as well as the voice actor):
|Game||Character||Face model||Voice actor|
|L4D1||Bill||Bernard Fouquet||Jim French|
|L4D1||Francis||Taylor Knox||Vince Valenzuela|
|L4D1||Louis||Sean Bennett||Earl Alexander|
|L4D1||Zoey||Sonja Kinski||Jen Taylor|
|L4D2||Coach||Darnell Rice||Chad Coleman|
|L4D2||Ellis||Jesy McKinney||Eric Ladin|
|L4D2||Nick||Taymour Ghazi||Hugh Dillon|
|L4D2||Rochelle||Shanola Hampton||Rochelle Aytes|
The body is easier to model in 3D, and is probably based on sketches made by Valve artists. However, it is very difficult to accurately model a face, and that's why they use real-life models for faces. (but, of course, they fine-tune the modelled face to fit better to the character, like the G-Man from Half-Life series)
It appears that they are just stereotypes:
There are four playable human characters in the game: Francis, a stereotypical biker (voiced by Vince Valenzuela); Bill, a Vietnam Veteran (voiced by Jim French); Zoey, a university student (voiced by Jen Taylor); and Louis, a district Account Manager (voiced by Earl Alexander).
However, I wouldn't be surprised if each characters appearance wasn't based on a real-life person - but that person could be anyone, even one of the developers!