Good question, I'm wondering exactly the same, I tried driving down the motorway thinking the background noise would be great for EVP... I placed my digital dictaphone on the passenger seat next to me. When I first listened I was like wow this is amazing, but after almost an hour of carefully listening and documenting it suddenly began to dawn on me that the dictaphone had picke dup lorry CB chatter... "You just cut me up thanks Roy" was the decider along with another voice saying "come in". When I listened carefully I realised there was constant radio chatter in the background... no spooks or spirits this time...
So I am left with the same question as yourself... I've a book by Anabela Cardosa called Electronic Voices... I'll have to finish reading it...
Good Luck! If you discover anything let me know!