From: "Phil Payne" 

> WHAT!?  There's sufficient EM on A PUBLIC WAY to prevent a car from
starting?
> And just why does a running car have more resistance?  Diesel?

http://www.guardian.co.uk/uk_news/story/0,3604,862399,00.html

It triggers immobilisers.  It depends on the immobiliser and how it was
installed.  The dead simple variety break the starter motor solenoid
circuit, in much the same way as it's broken in a slushbox car that's not
in "Park".  Keep the engine running, and you'll be OK.  If the
immobiliser breaks other circuits, especially anything related to the ECU
or ignition, the car will stop there and then and not restart.

Bear in mind that radio-type immobilisers have an aerial wire to receive
transmissions from the keyfob - you may not know about it.  Most installers
lay it along the back of the dashboard.  That's what Fylingdales triggers -
it uses the same frequencies as most immobilisers and because of its pulse
encryption often wanders into keyfob's pseudo-encryption.

You don't need much power to do this - a keyfob can do it with a single
silver cell from 20 yards, so a directional beam from a ground installation
miles away needs to be only a few watts.

--- BBBS/NT v4.01 Flag-5