apart can be supported. Given that there is a fixed distance between the two photos (I believe they use sensors in the road so that the cameras always fire when cars are in the same position, regardless of speed), all they need do is see if the two photos were taken less than a certain time period apart. For example, if the sensors are set 26 feet apart, they would look to see if that second photo occurs less than 0.57 seconds after the first. From the data it seems that 26 feet is the distance they used in their calculation of the distance covered between the two photos (26 feet/0.44s = 40.3 MPH).
BTW - you might be interested in this story, which supports your method:
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