INV is close to INV
max
= 5 cars. Hence, a better estimate of the actual peak throughput
is
λ
= 5
/
167
.
9 cars per second,
assuming
that CT = 167.9 seconds is accurate for the 90
minute lunch rush. (The original QSR study actually indicates that this is CT for the time
windows 11am2:30pm and 4pm7pm).
A firstpass at a corrected estimate is the following:
(a) 5,400 seconds in 90 minutes
(b) Assuming INV = 5 cars when CT = 167
.
9 seconds, this implies 5
,
400(5
/
167
.
9) = 160
cars served per restaurant during that period at current service speed.
1
(c) 77.9 seconds, is what McD’s must shave off its average drivethrough time to hit its
90second target.
(d) At CT = 90 seconds, they could service
at most
5
,
400(5
/
90) = 300 cars in that period
on average, assuming INV = INV
max
= 5 cars at this new CT. This would be their
capacity (recall the ovens at Varsity Subs).
So they could service
at most
an extra
300

160 = 140 cars, combining the result with part 2.
(e) Assuming $5 revenue per car, this yields ($5)(140) = $700 extra revenue per day for
each restaurant.
(f) Assuming 300 good drivethrough days per year, this amounts to ($700)(300) = $210,000
in additional peak period revenue earned per restaurant annually.
(g) Across 13,673 locations, this totals $2.87 billion, or about 15% of their current annual
revenues of around $20 billion.
In general, all the numbers (except 77.9 and 5,400) stated in the figure must be multiplied
by INV
≈
INV
max
= 5.