The costs of traffic to a city go beyond travel delays: the impact of congestion spans wasted fuel, increased pollution (and consequent poor health) and lessened economic productivity – it’s estimated that Cairo traffic is responsible for the loss of 4% of Egypt’s GDP and New York loses up to USD $10 billion annually because of congestion. A logical step to reducing road traffic is to increase use of public transport over private vehicles. 148 cities across the world have a metro system and there are close to 540 lines in total. Together, they carry over 150 million passengers per day, at 9,000 metro stations across the world, over 11,000 kilometers of line infrastructure.

We’ve collected the data and analysed modes of transport in 11 big cities that have their own metro subway infrastructure across the world – Mexico City, Moscow, Sao Paulo, Paris, Tokyo, London, Seoul, New York, Cairo, Tehran and Bangkok – and find that the inverse relationship between the length of a metro line and ridership points towards the need to increase capacity through more kilometers of track in most cities.

Meanwhile, the relationship between road congestion and metro ridership generally follow opposite trends, suggesting that more metro coverage can almost guarantee less traffic on the streets.

When it comes to identifying which mode of public transport will best equipped to decrease traffic, cities should look to London, Paris, Tokyo and New York where the metro-to-bus-ratio sways in favour of the subway.

While cities with higher car-ownership-to-congestion ratio should focus on increasing cost of car ownership, tolls and fuel, cities like Seoul, Cairo, Tehran, Mexico City and Bangkok need to expand both their road network and metro coverage.

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*Some missing data has been extrapolated using related information.