Due to recent trends in society, enhanced car safety is strongly required. As one piece of environmental traffic information, the direction of approaching vehicles can be estimated from an analysis of sound sources. This gives information of approaching vehicles to drivers at intersections when visibility is poor. To prevent traffic accidents, it is necessary to detect the direction of approaching vehicles more accurately and rapidly. This paper describes a scheme that uses a uniform linear microphone array with four microphones to estimate the direction of approaching vehicles. We employ a correlation method to realize sound source localization and we propose a direction estimation method with cubic spline interpolation in Carter, G. C. and Abraham, P. B. (1980). Trials show that the proposed scheme offers good performance.