The Connecticut emissions test is conducted at speed.
The rear tires are driven over rollers and then driven up
to about the 25-30 mph you mentioned. The car is run on the test stand for about one minute and the results are recorded over the
next minute ( at least that's what I was told by the attendant).

And yes diesels were exempted until '95 or so when
our money hungry state figured to pick up a few
extra bucks. ( Connecticut Legislators never met a tax they didn't like). However they did do a decent thing in exempting diesel trucks over a certain weight.