A dormant 'cash mountain' marks a nadir for London's contactless travel card, but trouble has been brewing for some time.
Cities are expanding upwards and downwards, as well as outwards. With urban density also increasing, moving people efficiently around the city, often using ageing infrastructure, is quite a challenge.
Rioters were influenced by a shared anti-police identity – not just mindless violence.
Sensationalist warnings of further riots abound. But while many social inequalities remain, we can't say whether more widespread violence will follow.
Computer models can help planners deal with large groups of people but we need better insight into the psychology of crowds to make them accurate.