The formation and propagation of queues on urban freeways is an unavoidable result of the ever-increasing traffic demand. This paper presents the design, specification, implementation and evaluation of an infrastructure based Queue Warning system (QWARN) that is capable of detecting dangerous traffic conditions, i.e. crash-prone conditions, on freeways and delivering warning messages to drivers, in order to increase their alertness to these traffic conditions and ultimately reduce the crash frequency on urban freeways. This study utilizes measurements of individual vehicle speeds and time headways at two fixed locations on the freeway mainline. The Queue Warning system was implemented at the right lane of a 1.7-mile-long freeway segment of Interstate 94 WB where the event frequency prior to the systems installation was 11.9 crashes per million vehicles traveled and 111.8 near crashes per million vehicles traveled. The control algorithm assesses the dangerousness of the given traffic condition and responds with a warning result based on a multi-metrics traffic evaluation model. The system translates the warning result into readable messages and delivers them to the two sets of signs located upstream of the detection zone. A three-month investigation of the operations of the QWARN system showed the event frequency reduced to 9.34 crashes per million vehicle traveled and 51.8 near crashes per million vehicle traveled. The result shows a 20% decrease of crash frequency and the feasibility of the proposed methodology.