It was impossible to flag a cab when you were invisible. Not that Reed minded-- actually, were he to get his choice and guilt, responsibility, and perfectionism not part of Reed's vocabulary, he would choose Sue's powers over any of the others of the Fantastic Four. Though, in the end, it was probably best that things had worked out as they did, or else Reed may have spent quite a fair amount of his time invisible.
Thankfully, they didn't have to walk but a few blocks before they'd lost the photogs and were able to turn visible again. Taxi hailed, the Richards made it back to the Baxter Building without any more unsuspected surprises.