The text message/email you are reading/composing is so absolutely, life depending, vitally important that you must continue to do-so whilst walking down the street.

You can normally do this by using the ‘walking at the speed of a stoned slug’ technique; where you meander down the street, totally oblivious to the tutting, frustrated commuters unable to overtake you on the busy pavement.

You can also optionally stop at random moments when your multitasking abilities run out, causing a minor pavement pile-up behind you.

(Thanks to cosmic_fish for the tip!)