Traffic data is the most important component of any transport GIS. They are mainly collected by traffic sensors (detectors). However, the installa- tion of sensors is quite expensive. There have been studies on the finding of optimum location of inductive loop detector, the most widely used sen- sor in the past decades. However, with the advancement of new sensor technologies in recent years, many new sensors are now available for multi-lane and multi-direction traffic monitoring. Most of them are nonin- trusive sensors which are more suitable be located at road junctions instead of roadways. Thus, different from previous studies on link-based sensor location, this paper explores method in determining critical road network junctions for the optimum location of nonintrusive sensors for monitoring and collecting real-time traffic data. The objective is to select the least number of junctions while maximally cover the road network. Since the problem is NP-complete, a greedy-based heuristic method is proposed and a numerical experiment is conducted to illustrate its efficiency.