The evolution of hydrodynamic and statistical parameters along the pipe was studied experimentally in gas-liquid slug flow for various flow conditions and two pipe diameters. The measuring modules comprise a set of three adjacent optical fiber probes and could be easily transferred to various positions along the pipes. The probes detect the passage of the gas-liquid interface. This technique enables one to measure the instantaneous velocities of nose and tail of elongated (Taylor) bubbles simultaneously with the slug length ahead of each bubble. The liquid slug and Taylor bubble length distributions along the pipe, together with the dependence of the Taylor bubble velocity on the liquid slug length ahead of it, are presented at various locations along the pipe. Empirical correlations relating the Taylor bubble velocity with the bubble separation distance are suggested. These correlations are used as an input to a model for slug length distribution. The model results are compared with the experiments.