How do you Know When You’re Too Close to the Rider in Front?

I always loved those bumper stickers that say, ‘If you can read this bumper sticker you are following too close.’ Nobody like a tailgater on the freeway. It’s the same on the trail.

Over the past couple of years we’ve had to learn new terms like social distancing, and re-calibrate our sense of how close is too close. Of course proper spacing depends on speed, relative speed, and terrain so for this question, assume riding an average trail at average speed.