I'd never heard of a deer whistle, so I googled:

There are a lot of people selling them, but the other 3 articles I
spotted were:

http://www.ibmwr.org/otech/deerw.html
http://www.advance.uconn.edu/2002/021118/02111812.htm
http://www.cnr.usu.edu/crossings/pubdetail.cfm?projname=&locstate=&Submit=Search
&offset=51&pID=862

There seems to be some doubt as to whether they actually work.

Andy

Use it as a hood ornament.....a very retro look that will be sure to turn
heads.

Going to the mountains—need any bear bells as well?
What is the difference between a black bear and grizzly scat?
Grizzly scat has bear bells in it.

re: FYI

The largest auto insurance company in the U.S. was reportedly
previously giving away deer whistles upon requests, but apparently  no
longer is doing so.

I do not know why they stopped giving them out.

It would seem to be their  interest to do so.

I've previously bought the little plastic noise/whistle things for $1
each to about $8 each.

They sort of mount on bumpers, but then seem to get knocked off from
the wind, car wash, et cetera.

I've got a vehicle that I glued 5 on (a "buck" each), and two or three
are still there.

Here is ad copy, which may be accurate:

"Every year there are more than 1.7 million reports of automobile
collision with deer. This number represents accidents where a police
report has been made to verify vehicle damage or personal injury. It is
estimated there are over 2 million unreported deer/vehicle accidents
each year, totaling an astounding 3.7 million vehicle contacts with
deer annually in the continental US. This staggering number accounts
for more than a billion dollars in insurance claims annually."

"In certain areas of some states, your odds of having a deer/vehicle
accident may be less than 1 in 3 within a year, with average vehicle
damage exceeding $2,800, plus risk of personal injury."

"The (name of a company), used with driver awareness, will reduce your
risk of hazardous animal/vehicle collision by more than 70%."