I'm prepared to concede that there may be some long-term benefits to be gained, e.g. in reducing the amount of maintenance work carried out over the lifetime of a project.

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!