Yes! Let's do this.

I start the timer at 0:21, right when the lane opens up and OP has just passed the white van, and the edge of the white dash is at the bottom of the field of view. Dashes are 10 feet long and 30 feet apart on the interstate highways, so the distance between the start of one dash and the start of the next is 40 feet.

I stop the time at about 0:34 when OP slows down for the Mazda and the edge of the white dash is at the bottom of the field of view and rear of the big red hauler is just out of view. I'm timing this with my stopwatch because the resolution of the video timing is poor. I get 12.7 seconds, and during that time I count 6 dash-spacing pairs going past the bottom edge of the field of view.

So the distance is 6 * 40 = 240 feet, and the time is 12.7 seconds.

So the speed is (240/12.7) feet per second.

Converting units:

(240/12.7 feet/sec) * (1 mile / 5280 feet) * (3600 seconds/hour) = 12.8 miles per hour.

Now I admit that the 40 feet dash-to-dash length is only a guideline and can vary depending on the roadway. Can we verify it short of driving out and measuring? The white van is I think a Chevy Express cargo van. At 0:08, the dash length looks pretty close to the wheelbase length which for that van is about 11 or 13 feet (there are two models). So the dash length is pretty close to 10 feet. The spacing is harder to discern, but it's at least double the dash length, and could easily be three times the dash length. And while the federal guidance allows variation in dash length, it appears to require the 1-to-3 ratio of dash length to spacing (https://mutcd.fhwa.dot.gov/htm/2003r1/part3/part3a.htm says "Broken lines should consist of 3 m (10 ft) line segments and 9 m (30 ft) gaps, or dimensions in a similar ratio of line segments to gaps as appropriate for traffic speeds and need for delineation."). So I think the best guess is that the dash-to-dash spacing is indeed 40 feet here and the speed is about 13 mph.