Transport in the United Kingdom is facilitated with road, air, rail, and water networks. A radial road network totals of main roads, of motorways and of paved roads. The National Rail network of 10,072 route miles (16,116 km) in Great Britain and 189 route miles (303 route km) in Northern Ireland carries over 18,000 passenger and 1,000 freight trains daily. Urban rail networks exist in Belfast, Birmingham, Cardiff, Edinburgh, Glasgow, Liverpool, London, Manchester and Newcastle. There are many regional and international airports, with Heathrow Airport in London being one of the top ten busiest in the world. The UK also has a network of ports which received over 486 million tons of goods in 2019. Transport is the largest source of greenhouse gas emissions by the United Kingdom. Transport trends Since 1952 (the earliest date for which comparable figures are available), the United Kingdom saw a growth of car use, which increased its modal share, while the use of buses declined, and railway use has grown more slowly. However, since the 1990s, rail has started increasing its modal share at the expense of cars, increasing from 5% to 10% of passenger-kilometres travelled. This coincided with the privatisation of British Rail, but its impact is heavily debated. In 1952, 27% of distance travelled was by car or taxi; with 42% being by bus or coach and 18% by rail. A further 11% was by bicycle and 3% by motorcycle. The distance travelled by air was negligible. By 2015 83% of distance travelled was by car or taxi; with 5% being by bus and 10% by rail. Air, pedal cycle and motorcycle accounted for roughly 1% each. In terms of passenger-kilometres, slightly over 662 billion were made by cars, motorcycles vans and taxis, 78 billion by rail, 39 billion by bus, 5 billion by pedal cycle and 9 billion on domestic air flights. Passenger transport has grown in recent years. Figures from the Department for Transport show that total passenger travel inside the United Kingdom has risen from 403 billion passenger kilometres in 1970 to 793 billion in 2015. Freight transport has undergone similar changes, increasing in volume and shifting from railways onto the road. In 1953 89 billion tonne kilometres of goods were moved, with rail accounting for 42%, road 36% and water 22%. By 2010 the volume of freight moved had more than doubled to 222 billion tonne kilometres, of