The motorists behind Sam honked their horns and cursed his slow speed then, when oncoming traffic cleared, they zipped around him, shaking their fists.
A. speed, then, when...
B. speed. Then, when...
C. speed then, fortunately, when...
D. no change
B. A and C would make the sentence a run-on. B is the only way of correctly writing the sentences. Although B would make two different sentences (the motorists... speed. Then, when oncoming.... fists), they are both grammatically correct.
