I need a fairly reputable number for the amount of time Americans
spend in their car each year.  I've heard numbers like "3 hours per
day" but I don't have a source for that.  Ideally, I would like an
aggregate number (e.g., 150 million Americans spend an average of 3
hours per day in their car leading to 450 million hours of "drive
time" each day).  However, if an aggregate number isn't available I
can probably extrapolate.

Thanks!

Here are some figures that should be useful:

"The Roland Berger study, due in November but announced in time for
next week?s Convergence 2000 automotive electronics conference in
Detroit, says Americans spend 540 hours a year in their cars - twice
as long as Europeans do."

Auto Beat Daily: OCTOBER 13, 2000
http://www.autobeatdaily.com/pdfs/10-13-00.pdf

"In the western world, drivers and passengers spend a significant part
of their lives in cars. In Europe, the figure is 274 hours per person
a year, while in the US it is 541 hours a year.
 Sources: Commerzbank; Roland Berger report"

Financial Times: The future dashboard
http://specials.ft.com/ftit/june2001/FT3A72I0JNC.html

"Five hundred forty hours - that is the average time Americans spend
in their cars each year."

Roland Berger: Dial M for Mobile
http://www.rolandberger.com/expertise/en/media/publications/RB_Dial_M_For_Mobile_2000.pdf

"According to the U.S. Department of Transportation, Americans spend
more than 500 million 'commuter hours' per week in their automobiles."

Automotive Industries: The internet changed your life, but can it
change your oil?http://www.findarticles.com/p/articles/mi_m3012/is_7_182/ai_89816542

"The Intelligent Transportation Society of America, a Washington D.C.
'think tank' that supplies data to the Department of Transportation,
reports that there are more than 201 million cars in the U.S. today
and that Americans collectively spend more than 500 million hours in
their cars each week with an average commuting time of 80 minutes per
day and growing."

Business Wire: Clarity to Provide Superior Voice-based Interface
http://www.findarticles.com/p/articles/mi_m0EIN/is_2001_Jan_9/ai_68912284

Google search strategy:

Google Web Search: "americans spend" + "in their cars OR automobiles"
://www.google.com/search?hl=en&ie=UTF-8&q=%22americans+spend+%22in+their+cars+OR+automobiles

I hope this helps. Please request clarification if anything is unclear
or incomplete; I'll be glad to offer further assistance before you
rate my answer.

Best regards,
pinkfreud

This was exactly what I was looking for (well, actually I was hoping
that the number would be higher but that was hardly your fault! :)

Thanks!

Thank you for the five stars and the tip!

Estimates vary from study to study. If you're looking for a higher
figure, here's one:

"On average, Americans spend 2½ hours in their cars each day. 
(Yankelovich Monitor Study 2000)"

XM Radio: Fast Facts
http://www.xmradio.com/corporate_info/fast_facts_mkt.html 

~pinkfreud

Great follow up.  Thanks!