How close do I have to be for effective drafting (while commuting)?
My opinion:

I don't believe you can effectively draft and have enough time to stop.  I don't know the exact aerodynamics (and I suspect it's affected by speed and wind), but if you watch any paceline or peleton, they're never more than a couple feet (about half a meter) behind the rider in front, often only a few inches (centimeters) behind. With a crosswind or larger groupings, they're even likely to have wheel overlap (where the front part of the front wheel of the following cyclist is next to the rear part of the rear wheel of the lead cyclist).

You can really only draft safely with the cooperation of the lead cyclist, who needs to signal back to you before starting to stop.

Do the math:

Let's put it another way:


If you're going 15 miles/hour (25 km/hour)
That's also 22 feet/second (6.9 m/second)
And if you're following 3 feet behind (1 meter) (a bit far for drafting)
You have 0.136 seconds (0.145 if you take the metric rounding version) to react to something that causes the lead bike to need to brake suddenly to avoid colliding.
If you're 2 feet (2/3rds of a meter), you have less than a tenth of a second.  If you're 2 feet and 20mph (32 km/hour) you have about 1/14th of a second to react.


Less than a sixth of a second to figure out that the lead bike is braking and to start braking, and that's assuming your brakes are as good or better than the lead bike's brakes.  If the lead bike has slightly better brakes (or wider tires, or grippier tires, or even just pulls the brake levers a bit harder than you do), you may simply have no reaction time available and a collision will be unavoidable.

In other words: you definitely can only safely draft with the full cooperation of an experienced lead cyclist who won't brake unless they've warned you or being rear-ended by you is better than whatever they're about to run into.  If you were going to go by hazards, they'd need to warn you about upcoming hazards and you'd need to pull back to 10 feet (3 meters) or more.

Avoid the door zone

Additionally, you should simply stay entirely out of the door zone so that opening car doors aren't much of an issue.  It's not unheard of for car doors to open very fast with no warning, so that nobody has enough time to avoid hitting the suddenly opened car door. I haven't checked the laws of every single state in the US (and definitely not outside the US), but in general the area that parked car doors can open into (the door zone) constitutes a hazard that you're not required to ride through.  When a bike lane is in the door zone I'll either ride the line on the left of the lane (if that's outside the door zone), or simply ride in the right wheel track of the car lane next to the bike lane.