Egress is measured in "people per minute." The short answer to your
question, with an impeccable source --is this:
26 people per foot (width) of door, per minute.
Calculate rate of egress:
Stairway rate 18 people/foot/minute.
For a 4 foot wide stair the rate is 4 feet x 18 people/foot/minute
= 72 people/minute
Doorway rate 26 people/foot/minute.
For a 3 foot wide door the rate is 3 feet x 26 people/foot/minute
= 78 people/minute
YOUR SOURCE IS:
INTERNATIONAL BUILDING CODE ? MEANS OF EGRESS (top right of page 1)
Density figures and calculations from:
SFPE Handbook of Fire Protection Engineering
Technically, this is the end of your Answer, however I'm passing along
all the other resources I found along the way. Note at Evacunet (not
as good a source), they measure "headway" :
EVACNET4 USER'S GUIDE ? DOORS
..."Headway is an important idea relevant to modeling the flow
capacity of doors; headway is the average time between two successive
users of a door. If the headway is too small, then successive users of
a door would interfere with one another.
As an example of headway, if the headway of a door is 1 second, then
the average time between successive users is second. Dividing 60
seconds per minute by the headway of a door gives the average flow
capacity of the door in people per minute. A door with a headway of 1
second has a flow capacity of 60 people per minute.
Fruin  points out that "Entrances are, in effect, walkway sections
in which pedestrians have been channeled into equal, door-width
traffic lanes." This remark suggests that, if carefully used, flow
volume for walkways may be applicable for doors.
Fruin  gives data for several entrance doors. For free-swinging
entrance doors, he has observed that average headway is 1.0 and 1.5
seconds, giving 40 to 60 people per minute being able to use the
doors. In the case of a door revolving in one direction, he has
observed that average headway is between 1.7 and 2.4 seconds, giving
25 to 35 people per minute being able to use the doors..."
Means of Egress
NFPA 101B, Code for Means of Egress for Buildings and Structures, 2002 edition
ADA Accessibility Guidelines for Buildings and Facilities (ADAAG)
Hope this helps! If you need clarification, please don't hesitate to ask.
Search strategy used at Google:
"people per minute"