I would guess as a delivery driver he may have had a fitted mobile phone with earpiece, though how that could have covered the siren noise.
Having done some delivery work, I know that most of the companies push van drivers to cover many deliveries / distances that I would be pushed to cover in a car, which is more agile & can legally travel faster. One route I drove for many weeks had the same timings no matter what size/type/weight of van (increased loading time) and therefor what the max permitted speed was on a single carriageway of 150 miles. Got many a bollocking for getting back late.
If the company managers/directors were also prosecuted, then delivery drivers would be a lot better drivers, either by the companies employing a different type of person or the drivers knowing care was more important than speed & money.