TRANSYT is a method of finding, and studying, the 'best' fixed time plans with which to coordinate the traffic signals in any network of roads for which the average traffic flows are known. This report summarises the principles of the TRANSYT traffic model and of the signal optimiser, and gives details of the data input and output formats for version 8 of the computer program which implements the method. A worked example is included. The TRRL program TRANSYT/8 includes several improvements compared with versions issued earlier: a preliminary output is provided to aid the selection of cycle time and single/double cycle decisions; a priority control model allows give-way junctions and opposed right turning traffic stremas to be modelled; an option is provided to reduce the likelihood of excessive queues occuring on selected sections of road; the output is substantially modified to include the monetary costs of delay and stops and to give estimates of fuel consumption. The program is written in Fortran IV and can be obtained under licence from TRRL