The bridge that Tod is sitting under fishing and the bridge that the wedding party drives over are not the same bridge, as the superstructure ironwork is different in the beams and girders. Tod is sitting under the St. Charles bridge for Hwy 115 and previously US 40, but the wedding party may be over the Blanchette Bridge which is the new US 40 bridge as of 1958.
While Tod talks to the police officer directing traffic, a 1960 Chevrolet and 1959 Ford wait in the bridge traffic to turn. Although the officer holds the traffic for Tod to drive through, the cars turn into a Ford Falcon with a Ramble station wagon behind it.
When Tod is talking on phone to Buz he implies that the echovirus makes him hear echoes. That is not a symptom for echovirus, and it appears the show's producer/writer simply didn't know what it was and just assumed that's what it meant.