Methods and systems suitable for negotiating air traffic trajectory modification requests received from multiple aircraft that each has trajectory parameters. The methods include transmitting from at least a first aircraft a first trajectory modification request to alter the altitude, speed and/or lateral route thereof. A first conflict assessment is then performed to determine if the first trajectory modification request poses a conflict with the altitudes, speeds and lateral routes of other aircraft. If a conflict is not identified, the first trajectory modification request is granted and the first aircraft is notified of the first trajectory modification request being granted. Alternatively, if a conflict is identified, the first trajectory modification request is not granted and the first aircraft is notified thereof. If the first trajectory modification request was not granted, the first trajectory modification request is placed in a queue, which is periodically processed to perform subsequent conflict assessments.