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Ramp metering is known as a technique of Intelligent Transportation Systems (ITS), which can improve freeway throughput and reduce the incidents with low implementation cost. However, its benefit of application is only effective in certain conditions. It is important to accurately analyze the benefit-cost ratio before the decision making of implementing it. Sketch planning tools provides a measure to conduct benefit-cost analysis based on the impact of ramp metering, such as throughput increase and safety improvement. The default impacts used in software are reported in previous studies and expected to be functions of the conditions under investigation and may not be transferable between transportation systems. This study proposes a method to estimate the impacts of ramp metering for variable freeway traffic conditions through modeling the reduction of traffic breakdown due to ramp metering. The results show that this procedure allows flexible benefit-cost analysis for various traffic conditions.