This may sound obvious, but how about a leash?  If the leash is a chain heavy enough to cause a dependable caternary bend, then it will not be necessary to wait for a pull on the chain to slow down.  Just look for the angle of connection to stay within a range.  Stop if the leash drops completely or does pull.  Slow if the leash starts to get horizontal, this indicates that further speed will result in pulling.

Another idea is an infrared led pendant that blinks at say 50 kilohertz.  The follower would wear this.  The blink provides noise immunity.  Then have two vertical sensing scanners at different heights and triangulate.

Eric Ratliff
http://www.icpdas-usa.com

Toro self-propelled lawnmowers use a system to control how fast to move the mower based on how fast the person walks behind it.  The person grabs onto the handlebar and pushes.  The handlebar is on a slider mechanism, somehow connected to the belt system that applies power to the wheels.  The faster you walk behind the mower, the faster the mower moves.

Maybe some variation of this idea would work:  mechanical, or LVDT, or cheap sensor to gauge the +/- desired speed of the person versus a pre-determined setpoint.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com