I was told that years ago there was a device used by the police to
measure a car's speed on the roadway.It involved a hose across the
road. Is there now or was there ever such a device??

Yes, there is and was such a device!

"HOW DO OFFICERS USE HOSES TO DETERMINE VEHICLE SPEED? 
An officer places two rubber hoses across the road. The hoses are like
those at gas stations that ring a bell,except these hoses connect to a
box that measures thetime it takes for your vehicle to travel between
the hoses. Your vehicle speed is indicated on a receiver in the police
vehicle. Since the hoses are not easy to see, are only a few feet
apart, and are used for collecting other traffic information,this can
be a very effective method for determining vehicle speed."

There is futher explanation and illustrations in this pdf file:

http://support.tandy.com/support_auto/doc66/66712.pdf.

Sneaky aren't they? :-)

-K~

Search terms: Car speed detection

very quick and very informative answers. Thank you!

Another site with lots of information about speed traps:

http://www.ukspeedtraps.co.uk/

For the kind of measuring device you're talking about, click on the
'Non Radar/Laser' link.