Improving the merging safety issue on freeway ramps is a major concern for transportation management operators due to the high costs of accidents. The goal of this research is to test a risk pre-control threshold for setting ramp metering by considering lane change safety during merging. In this study, the minimum safety gaps needed for merging are identified by the kinematic analysis of vehicles' braking process. The impacts of the difference between vehicles' speeds to the shape of safety gaps have been analyzed. The density function of the risk pre-control threshold is deduced and the distribution is obtained by numerical simulation on the basis of NGSIM dataset. The results show that the merging vehicles' accelerations greatly influence the selection of safety gap by drivers, and the threshold shows it is not a definite value but a range, so the application condition is discussed. The findings of this study can be used to complement existing ramp metering strategies and realistically help detect or reduce the safety risks in merging areas.
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