This thesis describes the research undertaken in the definition, coding, creating, validation and application of a microscopic simulation model, TRGMSM (road Traffic and Rail vehicles General Microscopic Simulation Model), which is for the investigation of grade operation of LRT (light rail transit) at signalised intersections. This simulation model has the following main features: - Geometric Aspects: Cross roads and T-junctions can be simulated with a maximum of 4 lanes on each approach in addition to light rail tracks located in a central reserve. LRT stations may be located up-stream (USS), down-stream (DSS) or mid-block (MBS). - Characteristics of Vehicles and Drivers: Each vehicle and driver is identified with more than twenty characteristic and behaviour parameters such as vehicle type, desired speed, car following, gap acceptance, lane changing, braking reaction, amber reaction etc. which are randomly generated when the vehicle is initially generated. - Traffic Signal and Light Rail Vehicle Priority Measures: Seven types of commonly used LRT priority measures have been modelled under various geometric, signal conditions. - Visual Display: Simulation process can be visually displayed as the simulation progresses. The time and distance relationship for vehicles in a selected lane is continuously displayed at the bottom of the screen. Application results of the model are presented to illustrate the way in which the following factors may influence the at-grade operation of LRVs and road vehicles: i) With and without at-grade operation LRT,ii) Variations of LRT headways and traffic demands,iii) Location of the LRT stations,iv) Signal control and LRT priority alternative,v) Turning traffic restrictions in junction. The results revealed that properly designed LRT priority measures can significantly increase capacity (in persons per hour) and reduce person delay with only insignificant extra vehicle delay in junctions. The discussions also include guidelines for at-grade LRT design.