Most of the Dutch motorway system has such lane signalling since the 1970s, combined with something called 'automatic incident detection' (AID). It automatically shows flashers and a lower speed when induction loops detect slow or stationary traffic. It is very effective and has proven to significantly reduce the number of crashes near the end of a traffic jam. The flashers are only shown in the slowdown phase where traffic has to slow down from 130 to 50 km/h, it doesn't show flashers all the way through the traffic jam.
Germany doesn't have this system and they have trucks plowing into traffic jams resulting in fatalities or serious injuries more than 300 times per year (basically every day when trucks are not banned from driving...)
I'm 99% sure Washington picked up the idea of
per-lane active traffic management from either the UK or the Netherlands. According to
this video from WSDOT (which shows off the ATM system along the M42 near Birmingham), the system uses embedded roadway sensors, in addition to humans, to automatically detect slowdowns and change the signs (which can display
quite a lot of different symbols). According to WSDOT's
Grey Notebook, the ATM section of I-5 south of Seattle was (from 2007-on) the only stretch of freeway in the Seattle area to see a reduction in collisions (except those that also have had ATM systems installed).
I'm really surprised Germany hasn't introduced ATM systems. You'd think the Germans, which many regard as the kings of speed, would be all over a system like this, especially since they are mental about safety (always staying right, slowing down during rain, etc).