The development of slug flow along vertical pipes is governed by the interaction between consecutive elongated bubbles. It is generally assumed that the trailing bubble's shape and velocity are affected by the flow field in the liquid phase ahead of it. To examine this assumption, a facility is used that allows controlled injection of pairs of Taylor bubbles into vertical pipes filled with stagnant or flowing liquid. An experimental approach is developed to perform particle image velocimetry measurements of the velocity field in front of the trailing Taylor bubble, simultaneously with video imaging of Taylor bubble pairs, to be able to relate the instantaneous parameters (shape and velocity) of the trailing bubble to the instantaneous velocity distribution in the liquid ahead of it. Experiments are performed in pipes of two diameters and for a number of Reynolds numbers based on mean water velocity, corresponding to both laminar and turbulent background flows. A model relating the propagation velocity of the trailing Taylor bubble to the local mean centerline velocity of the leading bubble is suggested and verified experimentally. The effect of the velocity fluctuations in the leading Taylor bubble's wake on the instantaneous propagation velocity of the trailing bubble is studied.