Not simply a measurement of shoulder width, selecting a handlebar width for a given application depends on a rider’s focus, mobility and handling preferences.
For road riders, a bar near to their shoulder width provides a balance between comfort, aerodynamics and leverage, which may be required for long or difficult climbs.
A Cyclo-Cross or Gravel rider may choose a slightly wider bar to provide more stability and leverage over the steering axis.
Meanwhile a Track Racer will likely benefit from the aerodynamic advantage of a narrower bar.
The increased availability of flared bars means that some riders can find the best of both worlds in some cases.
AC Performance Bike Fit 