Where, when, how did the habit of using the double indicator on cars to signal
traffic jam come up?

I use it to draw attention to the fact that I'm suddenly slowing down
on a high-speed road and I don't want the Starbux-swilling, BK-eating,
smoking, cell-phone-talking, satellite-radio-tuning, DVD-watching
"driver" to plow into me.

At 110kph (65mph) you aren't so much driving as aiming.

Strange. I have lived in two major Canadian cities and don't think I
have ever seen this. I wonder if it is a local thing?

Then again, I am known to be a BK-eating, smoking, cell-phone-talking,
satellite-radio-tuning "driver" myself. :P

I've never heard of this either. I have wondered how flashing one's
headlights to indicate there is a cop ahead, started. That I have seen
my entire life here in the US. I have not seen the double indicator
thing though.

 - artqs

I have seen this too, artqs. Actually, I think that this all comes
from a truck driver "code". If you watch truck drivers on the highway,
when one truck is passing another, the truck being passed will flash
their lights when the vehicle in front of them is safe to re-enter the
lane. The truck driver who passed them will then flash a thank-you.
Similarly, when a truck driver is re-entering the highway (especially
on an upwards sloping hill), they will often enter using their
double-indicator lights to indicate to other drivers that they are
driving slower than usual, since it takes them longer to speed up.

So, I think that all of these things: double-indicator to indicate
slow traffic, flashing for police, etc. probably originate from some
professional truck drivers code.

I rather think the above has little to do with emergency blinkers.
Here is a website for retro-fitting one to older Volvos that said that
they started to appear in the 1960s, but were not standard equipment.
http://sw-em.com/emergflash.htm

This site is more specific:  "post-1965 cars"
http://www.secondchancegarage.com/public/284.cfm

This site agrees, mentioning them on 1966 T-birds:
http://www.thuntek.net/cougars_unlimited/tbseqts.htm

In Europe they are also used to warn about freeway traffic jams, that
one is not just slowing down behind another moving car, but coming to
a stop or very much slower speed.  They are required for both vehicles
when towing, and when stopping in the breakdown lane, and generally
used when the vehicle is standing somewhere that could be a hazard to
traffic and to signal that the vehicle is parked, e.g., double
parking.

Here in the UK when drivers first started using their hazard lights to
warn motorists on the motorway of the danger of a traffic jam ahead,
the police and motoring organisations warned that the lights should
not be used for that purpose. However, the practise became wide-spread
and it was realised that it did help to prevent accidents and warn
drivers to slow down as there was a blockage ahead.

It has now entered the Highway Code and Law.

"96: Hazard warning lights. These may be used when your vehicle is
stationary, to warn that it is temporarily obstructing traffic. Never
use them as an excuse for dangerous or illegal parking. You MUST NOT
use hazard warning lights whilst driving unless you are on a motorway
or unrestricted dual carriageway and you need to warn drivers behind
you of a hazard or obstruction ahead. Only use them for long enough to
ensure that your warning has been observed."
http://www.highwaycode.gov.uk/08.htm#96

Road Vehicles Lighting Regulations 1989 
Section 27 section 5
http://www.opsi.gov.uk/si/si1989/Uksi_19891796_en_4.htm#(Tiii)27restrictionuselampotherthanthosewhichregulation24refer

I would say it is impossible to find out who was the first to use the
hazard lights on the motorway for this purpose.

answerfinder-ga

Thank you all for the tips, and I 
guess jennypoo probably is on the right 
path, if such code exists / ed I would 
accept that as an answer.