There are over 4,500 fatal intersection crashes each year in the United States. Intersection Advanced Driver Assistance Systems (I-ADAS) are emerging active safety systems designed to detect an imminent intersection crash and either provide a warning or perform an automated evasive maneuver. The performance of an I-ADAS will depend on the ability of the onboard sensors to detect an imminent collision early enough for an I-ADAS to respond in a timely manner. One promising method for determining the earliest detection opportunity is through the reconstruction of real-world intersection crashes. After determining the earliest detection opportunity, the required sensor range, orientation, and field of view can then be determined through the simulation of these crashes as if the vehicles had been equipped with an I-ADAS. This study reconstructed 40 Straight Crossing Path (SCP) and Left Turn Across Path Lateral Direction (LTAP/LD) crashes using pre-crash speed data extracted from the event data recorder (EDR) in both vehicles. The EDR contains the actual pre-crash speed of the vehicle prior to the crash, and can be used to determine the kinematics of the vehicle prior to impact. The crashes were then simulated as if either vehicle had been equipped with an I-ADAS sensor (i.e. a side-facing radar located on the front of the vehicle). The ability of the I-ADAS sensor to detect an oncoming vehicle was evaluated at the instant the first vehicle entered the intersection, which was defined as the earliest detection opportunity. For one-half the crashes analyzed, the Time-to-Collision (TTC) at the earliest detection opportunity was less than one-second. The distance between vehicles was less than 50 m at the earliest detection opportunity for nearly every crash. The angle from the vehicle heading to the oncoming vehicle varied by approximately +/-30° at the earliest detection opportunity. An intermediate beam radar (90 m, +/- 30°) directed at 43° from the vehicle heading was found to be capable of detecting the highest proportion of approaching vehicles These results have important implications for designers of an I-ADAS who are considering the necessary range, azimuth, and orientation for detecting an approaching vehicle.