If one is traveling on a highway that runs through populated areas for the entire span of the trip, is Tefillas HaDerech recited? If so, at what point during the journey is it recited?
The Mishna Berurah (110:30) and the Aruch HaShulchan (Orach Chaim 110:11) rule that one should recite Tefillat HaDerech when riding on a train,The risk was being attacked on the train.Therfore it would seem that the city limits are not the issue.The Modern day Pokim go from one end of the Spectrum to the Other.
Reb Shlomo Zalman (Minchat Shlomo Tinyana 60:4)says say it with out the Name of Hashem as Since there are always people on the way, therefore you are more secure, This is in refrence to Tfillas Haderech between Bnei Brak and Jerusalem.(Another option I have Heard quoted is to say it within the bracha of shema koleinu during the last tefilah before your trip.)
Reb Sternbach(Teshuvot V'Hanhagot 1:199) says that since car accidents are so regular you should have to say it every time you go into the car ,just the tefilah was made for people leaving the city thats when we say it.