The berachah of Tefillas HaDerech was enacted for someone travelling outside the city. If one has decided in his heart that he is leaving the city, the Mishnah Berurah brings a machlokes if one can recite Tefillas HaDerech whilst still in the city. He concludes that practically one should wait until getting out of the city.
The Chazon Ish said that nowadays many accidents happen inside the city, therefore, one can even recite the berachah whilst inside the city, therefore, if one recites Tefillas HaDerech inside the city, “don’t shout or scald him”.
It’s important to note that the Chazon Ish never said that doing so is ideal, he simply said that if one does so he doesn’t lose out and he has what to rely on. However, ideally one should wait until he leaves the city.
According to the above, if one said Tefillas HaDerech at the beginning of the journey, since according to the Chazon Ish one who says it then has what to rely on, it certainly wouldn’t be considered a berachah levatolah. (R’ Yishai Kohn shlita, was me’orah me to this point.)
