The traveler may pray the travel (shortened) prayer once he has passed the city limits
Question

                What is the ruling if a person starts traveling after the time for Zuhr begins, and after traveling for approximately ten kilometers he stops to pray? Should he pray the complete prayer (four rak’ahs) or the shortened prayer (two rak’ahs)?
Praise be to Allah. 
          The view of the majority of scholars is that the traveler 
          is allowed to pray the travel prayer (i.e., shortened) once he has passed 
          the city limits, because the Prophet 
          
          (peace and blessings of Allaah be upon him) used not to shorten his 
          prayers when traveling until he had left Madeenah. Then he would pray 
          two rak’ahs, because what matters is the time when the action is done. 
          If the muezzin gives the adhaan for Zuhr or ‘Asr, and the traveler sets 
          out and passes the city limits, then he is allowed to shorten the four-rak’ah 
          prayer. What counts is the time when the action is done, not the time 
          when he left the city, because at the time when he does the action he 
          is traveling.