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通行能力预测是公路设施规划、设计与运营中的基础环节.本文基于埃及曼努菲亚省12条农村双车道路段数据，研究了公路线形等几何特征对通行能力的影响以及线形由切线变为平曲线时通行能力的损失.所选取的路段线形包括直线和随后的平曲线.同时，采集了各路段上车流量及速度数据，利用基于基本图的外推法研究流量和密度关系.路段上不同车型的车辆均转换为小客车当量.此外，针对不同情形（切线、平曲线及对应流量损失），分别建立了最佳回归模型.结果表明，切线型道路的关键自变量是路宽、肩宽以及切线长；平曲线道路的关键变量是曲线半径和路宽.通行能力和几何特征关系最佳模型的自变量为曲线半径.本文的模型可用于分析评价农村双车道道路的通行能力，尤其是评价文中所研究路段的通行能力.

Effect of Highway Geometric Characteristics on Capacity Loss

The estimation of roadway capacity is essential in the planning,designing,and operation of highway facilities.This paper aims at assessing the influence of highway geometric characteristics on capacity at tangents and horizontal curves as well as on capacity loss at the change from tangent to curve.Traffic and geometry data obtained from twelve rural,two-lane road sites in Minoufiya Governorate,Egypt,were used.Each site consists of a tangent element and the succeeding horizontal curve.Vehicle flows and speeds were collected at each element in the study sites.A capacity estimation method that was based on extrapolation from a fundamental diagram which represented the relationship between traffic flow and density was used.The effect of different vehicle types was accommodated for by converting them into equivalent passenger car units.Regression analysis was used to investigate the relationships between geometric characteristics and capacity.The best regression models for each case(i.e.capacity at tangents,capacity at curves,and capacity loss between the two elements) were introduced.For tangents,the significant independent variables are lane width,shoulder width,and tangent length.In the case of curves,the significant variables are curve radius and lane width.The best model that exhibits the relationship between capacity loss and geometry characteristics includes only curve radius,which functions as an independent variable.The models are very useful and can be used to deal with capacity analysis as well as for the evaluation of rural,two-lane roads,especially for the area under study.