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Examining and improving the limitations of Gazis–Herman –Rothery car following model Name: Hamid Al-Jameel (Research Institute: The Built and Human Environment) Simulation models are effective tools to solve traffic problems because of their feasibility to represent the most complex situations. Different car-following models have been used to simulate traffic movements. The car-following model known as Gazis-Herman-Rothery (GHR) is one of the earliest models that have been used since the late 1950`s and up to this time. In this model, the acceleration of follower is based on the spacing and relative speeds between two vehicles (the follower and the leader). This model has many limitations, for example there is no response (or zero acceleration or deceleration) for the follower when the relative speed between the leader and follower is equal to zero for any relative spacing between the two vehicles. Another limitation includes the effect of the leading vehicle on its follower even if the distance between them is so large. This effect obstructs the following vehicle from increasing its speed to reach its desired speed and this reflects unrealistic behaviour. This paper tries to overcome the effect of large spacing due to car-following by using a spacing threshold to transfer the car-following regime to a free-flow regime. Finally, sensitivity analysis has been made to select the values for parameters in cases of acceleration and deceleration. The modified model has been validated by using real data. The parameters of the model have been selected and suitable values of spacing threshold have been used in calibrating the model