“You raise a valid point,” said Rabbi Dayan. “In fact, some achronim seem to limit this halacha to a per-diem worker who is hired for the entire day. Going to his assigned destination is included as part of his work hours.” (Avnei Nezer C.M. 52:4)
“Nowadays, when travel is usually not included in the work hours,” continued Rabbi Dayan, “one can question whether to consider coming to the workplace as beginning to work. Nonetheless, it seems the sages treated heading to work as beginning of work for this purpose. Therefore, the employer is liable for approximately half the amount if the worker does not find replacement employment.” (See Hayashar V’hatov, vol. 10, pp. 196-197)