Here's how I see it.
The EGR Solenoid is "on" all the time that the Duke is warmed up and running...kinda...
BEFORE the motor gets to operating temperature (131* F. and into closed loop) the EGR solenoid is "off"..and at 131* F. it is commanded to come "on" and it stays on, for a time.
This supplies full available vacuum TO the EGR valve..
But the O2 sensor can "see" the rich or lean and when the exhaust reaches a certain "smog level", then the PCM brings the EGR solenoid back "on".
The results happen nearly instantaneously, smog is reduced far faster than the O2 sensor can react...(the O2 sensor being rather slow) and it (the O2 sensor) "catches' the perfect 14.7 to 1 air/fuel ratio on the way (up or down) and controls the fuel mix accordingly. This never sets a code or the SES light to on, UNLESS the mix ratio never comes back into spec. It all happens that fast.
It is a delicate balance...the EGR valve sits there, just on the balance point of opening, hold close spring fighting a constant battle with exhaust pressure, ready to be commanded to open with a vacuum pulse from the solenoid. And it happens right now. Boom, it is open and Boom, it is shut.
But, at a sustained speed of some 40 to 45 MPH everything related to excessive smog production
is at a zero point. The engine is at a steady throttle state, with the EGR valve commanded "off".
Input from the VSS is monitored by the PCM, and it looks for this steady state at speed...this is the time that the PCM "tests" the EGR system to "see" if the EGR is there and working.
The PCM commands the EGR solenoid valve to "on" and watches the mix ratio signal from the O2 sensor. It is looking for an immediate change, more exhaust, less oxygen. After a time period (remember that the O2 sensor is slow), the O2 sensor sees the change and the EGR solenoid is commanded "off".
The reason that the EGR valve is tested at a high(er) road speed is that the stumble induced by the opening EGR valve is not so driver noticeable at speed...