However, writing detailed tests up front would completely destroy my "flow" when implementing features in a web application. It would certainly take me more time because there is going to be a large increase in the amount of code to write.
I simply don't have the time available to do this. I get a few weeks to knock out an application and then I'm on to the next thing!
In my experience, even if you are building a quick'n'dirty prototype, you'll always need to fire up a terminal or run the GUI to interact/play with the prototype you are building, to see if it "works".
In this case you may apply a --this is my very own definition-- "lightweight TDD approach" just to automate all the procedures that you normally do to test your prototype. Moreover, since you are writing a test for something that does not exist, it may help you defining better what you want from your experimental code, because you are using your prototype before it even exists :)
The massive upside is that there is much less defects in the end product (think it was 50-80% in the study, but can't recall exactly).
Depending on the project, the extra time needed, might or might not be worth it.
Take a somewhat complex web app, that is used for a marketing campaign. It has a life time of only 2 months. In this case it does not make sense to use TDD, as the lower defect rate and "future-proofing" qualities, might very well not be worth an increased budget.
As a counter example, I'm working with a company that uses TDD for their current development, but employees work on a platform with little test coverage. As a consequence of lacking unit tests, they're only able to release a few versions each year because their manual testing burden is so high (estimated by one manager as about half of the cost of a release). They've found that TDD for their new features doesn't escalate the QA cost as significantly as not doing it. Bottom line: it's a little more time up front, but they've found it to be a long-term gain.
Many programmers who begin TDD have the same concern as you. Many of them find that over time they're more productive over the long haul with TDD. It takes a little while to arrive at that realization.