Nevertheless, the "fix posting area" method was the main technique used in U.S. en route system prior to the 1960s, for example, when en route radar became more widespread. This method evolved from similar techniques used in railroad dispatch. It simplifies conflict detection because aircraft on different routes can only possibly conflict at intersection points with other routes, or with aircraft on the same route. Each intersection point ("fix posting area") in the sector is made into a separate bay on the strip rack, and multiple strips are printed for each aircraft, one per bay. The controller then sorts strips within each bay by the estimated time the aircraft is expected over that point. Only aircraft estimated to cross within 10 minutes of each other are considered possible conflicts, at which point altitudes are then compared to determine if there actually is a conflict. Aircraft on the same route however (in either direction) must be checked for conflict regardless of time over the intersection.