As the Location Sound Mixer, sometimes it's not dialogue I need to capture, but other sync sounds. Of course, often, some sound guys might lav up an actor who has no dialogue or boom from above when the only sounds in the scene are footsteps. These might be dumbfounded moments, but when an entire scene has no dialogue or any sounds from actor's mouths, you need to think what sounds do we want and what sounds don't we want?
We had other shots like this however, we did not use the follow/lead truck. We had the camera on a dolly on the sidewalk, catching the cyclists as they pass by. For this, I already had a lav mic on both actors, so I decided to leave those, and also use the boom. I brought in the lav levels as they got closer and closed them as they passed, catching their pants effectively at the right time and getting the perspective sound from the boom. I still would have liked another mic on the bike or a better way to avoid the moving shirt from occasionally bouncing and hitting the lavs, but it was for such a short moment, that we got what we needed from the lavs, or worse case scenario, adding "efforts" for this would be very easy to match.