The daf yomi is learning the sugya of תפילת הדרך now (yesterday and today berachos 29b and 30a) so I thought I would comment on some issues that come up relating to תפילת הדרך nowadays.
It is clear from the context of the gemara that תפילת הדרך is a תפילה בעת צרה. The gemara mentions תפילת הדרך right after it mentions that a person who is in a dangerous place says a תפילה.
The world has changed a lot since the time of chazal especially related to travel and we need to understand if these changes affect the din of תפילת הדרך. Here are some of the changes that have occurred relating to travel.
1. Most travel is no longer considered unsafe. When I get in my car every morning to drive to work I don't think twice about it
2. The distinction between in the city and out of the city is no longer true. Most people fell much safer driving in the Catskills then driving through Harlem. Driving on a highway from Brooklyn to Queens is in the city but driving in Monsey to the supermarket may be considered out of the city.
3. This is related to 1, people travel all the time. Most people commute some distance to work, this is normal and part of everyday life. In the time of chazal יוצא לדרך was a big deal.
Given the above R' Shachter (in Nefesh Harav) says that R' Soloveitchik did not say תפילת הדרך when he commuted from Boston to NY to give his shiurim. He felt that because it is a תפילה בעת צרה and nowadays there is no perceived danger and it is routine that there was no reason to say it.
The gemara states that you only say תפילת הדרך if the trip is 4 mil outside the city. Is this a measure of time or distance? Chazal use the word mil for both, sometimes distance sometimes time. If it is distance (as I believe the Mishna Berura holds), about 3 miles, then it comes out that if you live in Monsey and drive 5 miles to the supermarket you probably need to say תפילת הדרך (if you don't accept the Rav's opinion above). R' Ovadia Yosef holds that it is a measure of time. However, this is not so simple either. How do you measure the time? Is it how long this trip is going to take, or is it the average time? Imagine the following case. You are driving today from A to B in the rush hour where you estimate it will take 75 to 90 minutes. According to R' Ovadia you would say תפילת הדרך. Tommorrow you take the same trip in the middle of the day and you estimate that it will take only 1 hour. Do you say תפילת הדרך? It could come out that for the same exact trip 1 day you say it and one day you don't. צ"ע
R' Yakov Kamenitsky brings up the following point. It is clear from the Gemara that תפילת הדרך is a תפילה. The minhag seems to be that whenever you go on a trip with other people one person gets up and says תפילת הדרך and is מוציא everyone. This is fine for berachos, but for tefilla, someone else cannot be מוציא you if you are a בקי namely, that you can say the tefilla yourself. Nowadays we are all considered to be בקיאים, and therefore if תפילת הדרך is a תפילה someone else can not be מוציא you and you need to say it yourself. In fact, R' Yakov recommended that everyone should say תפילת הדרך themselves and he was against the common minhag of one person saying it because he felt that there is a real question of whether you are actually יוצא or not if only one person says it and you answer אמן.