How do they measure the distance of traffic jams?

"We use devices called 'traffic counters' which are build beneath the expressway to measure the distance,"

"That's the name for electromagnetic lines buried between five to ten meters beneath the expressway. When vehicles pass over, we can measure not only the number of cars, but also their size and how fast they're traveling."

"For example, in the case of the Toumei Expressway, there is a pair of traffic counters planted every 2km. So if we detecte any speed less than 40km/h (our official definition of 'traffic jam,') for two consecutive pairs of counters, then we know we have 4km of conestion; less than 40km/h for three consecutive pairs, than 6km of congestion, etc."

そんな仕組みで「ちゃんと」渋滞の長さって計測されていたんですね…。
じゃぁ一般道はどうなのよってのは？

街中のギモン Street Smart Trivia【日英対訳】 (対訳ニッポン双書)