It is crucial to appropriately deploy Roadside Units (RSUs) to guarantee the quality of communication service. The Delta Network metric △_ρ2^ρ1 is an important metric to evaluate the QoS of VANET. Based on this metric, Sarubbi, et al. proposed a deployment method Delta-r by using relative trip duration. However, the "false high" relative trip duration of vehicles at urban cells might play a negative effect on decision-making. In order to solve this problem, an improved algorithm Delta-uc, which is based on Useful Contribution, is presented. It avoids the negative effect of "extra" trip duration by retaining only the useful relative trip duration of a vehicle at an urban cell. The experimental data are coming from realistic mobility trace of Cologne, Germany. Experimental results indicate that in many cases of service requirement metrics, the Delta-uc algorithm can obtain fewer roadside units than the Delta-r algorithm, and is more practical in realistic applications.