Results of simulation studies are described: the studies were designed to determine how effectively left-turning drivers can be alerted to imminent conflicts with opposing traffic under difficult operating conditions and with limited detector capabilities. These conditions include approaching vehicles changing speed in locations not covered by detectors and detectors that may be able to detect only vehicle presence, but not speed. In cases without direct speed detection, one may try to rely on historical speed statistics to estimate the speed of approaching traffic, but unless the approach speeds are confined to a narrow range, the system is vulnerable to both false positive and false negative alerts in the respective cases of the real vehicle speeds being less than and greater than the assumed historical value.