Radio plays format

You may want to check with the radio station you're submitting to. There's a station in Chicago that has very specific requirements. According to the BBC, American scripts number the speeches and the BBC doesn't. I've written three radio dramas, the first one I followed the American format and it was a pain when you were trying to revise. (The BBC lets you download a macro-heavy program for formatting the scripts. It ran reasonably well with Word 2000, but had some peculiarities.) I can see some sense to the numbering -- locating a speech in giving notes, for example -- but it turned out my director didn't use that facility at all. So the last two I didn't number at all. Rule of thumb is supposed to be 1 minute per page. The BBC macros, combined with my tendency to write shorter speeches, ran a little shorter than that. Oh, and in timing, don't forget that wherever you have sound fx, that's treated as a person speaking. Everything stops, the sound fx goes, the dialogue resumes. Good luck.

re timing and FX, yes things stop - unless its something that starts and runs under - FX WIND, FX WATER SOUNDS, FX SEAGULLS etc - sometimes with radio scripts you can end up with two or three FX running under dialog.

The usual convention I've done is to say with the initial FX CREAK OF SHIPS WOODEN HULL (continues under) then let dialog run, and making a point of stating at the right moment (FX SHIPS TIMBER STOPS) - which usually means another FX abruptly starts as the next scene begins. Sometimes the FX have to 'dovetail' into each other to segue into the subsequent scene. Going from interiors to exteriors, so to speak.

Depends on the complexity and setting of the 'worlds' the story is in, as well as how much FX fun one wants to have
I was raised on UK Goon Shows, which pushed FX usage to the utter limit, so some of that rubbed off. Also 'Captain Kremmen' (British comedy radio series by Kenny Everett) and ditto, the famous 'Chicken Man' (which I think is USA product)