Not to remove any of the Cool factor of this one, but I'm not 100% certain he's after speeders. Looking closely at the lanes that he's set up right beside, following them a bit in each direction, and spinning the view at several points, these are definitely reversible lanes. They run one direction at certain times of day, and the other direction at other times.
Now, the most common reversible lanes like this are also HOV lanes. High Occupancy Vehicle lanes. Minimum 2 to 3 peopl in the vehicle if the vehicle is to be using the lanes. Violations punishable by large fines in most places tha have/use them. So it's quite possible, IMHO, that the cop is enforcing HOV restrictions instead of watching for speeders. Likely we'll never know what he was actually after. Still a cool find though.
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Land - Cars 
Now, the most common reversible lanes like this are also HOV lanes. High Occupancy Vehicle lanes. Minimum 2 to 3 peopl in the vehicle if the vehicle is to be using the lanes. Violations punishable by large fines in most places tha have/use them. So it's quite possible, IMHO, that the cop is enforcing HOV restrictions instead of watching for speeders. Likely we'll never know what he was actually after. Still a cool find though.