One of the great advantages of using a motorcycle to carry out courier work is its ability to get through traffic. As an example current journey times for 4 wheeled traffic getting through and about London are NO better now than what they were in Victorian times ie when a horse and cart were the main transport ‘vehicles’.
Filtering through city traffic by motorcycle can be done safely and legally. Of course there are dangers, but there are dangers all around where ever you go, whether on 2 wheels or more. Speeds, distance, looking both ahead and using peripheral vision are key factors to stay safe.
Keeping speeds relative to traffic/road conditions, keeping a safe distance from other traffic (and pedestrians!) in that you always have an escape route and constantly scanning the road ahead.