  The time delay between gravitational wave signals arriving at widely
separated detectors can be used to place upper and lower bounds on the speed of
gravitational wave propagation. Using a Bayesian approach that combines the
first three gravitational wave detections reported by the LIGO collaboration we
constrain the gravitational waves propagation speed c_gw to the 90% credible
interval 0.55 c < c_gw < 1.42 c, where c is the speed of light in vacuum. These
bounds will improve as more detections are made and as more detectors join the
worldwide network. Of order twenty detections by the two LIGO detectors will
constrain the speed of gravity to within 20% of the speed of light, while just
five detections by the LIGO-Virgo-Kagra network will constrain the speed of
gravity to within 1% of the speed of light.
