The smartphone environmental warning alert system would only need to be active during the time of walking, sensed by accelerometer motion, and during time of screen usage. That combination would be limited, although seems to be increasing.
(Especially evident when disembarking planes, additional disclosure on the patent would be to warn the user of an irate/impatient pedestrian behind them, although likely beyond the scope of the forward located device’s sensitivity.)
If this is ever reviewed by a patent examiner or legal professional, I contend that this invention is obvious and anyone skilled in the art would be capable of arriving at this solution.