  We propose a dynamic model for a system consisting of self-propelled agents
in which the influence of an agent on another agent is weighted by geographical
distance. A parameter $\alpha$ is introduced to adjust the influence: the
smaller value of $\alpha$ means that the closer neighbors have stronger
influence on the moving direction. We find that there exists an optimal value
of $\alpha$, leading to the highest degree of direction consensus. The value of
optimal $\alpha$ increases as the system size increases, while it decreases as
the absolute velocity, the sensing radius and the noise amplitude increase.
