responsible for trunk roads and motorways in England apart from the privately owned and operated M6 Toll. The Department for Transport states that traffic congestion is one of the most serious transport problems and that it could cost England an extra £22 billion in wasted time by 2025 if left unchecked. According to the government-sponsored Eddington report of 2006, congestion is in danger of harming the economy, unless tackled by road pricing and expansion of the transport network. In the year from February 2017 to January 2018, UK airports handled a total of 284.8 million passengers. In that period the three largest airports were London Heathrow Airport (78.0 million passengers), Gatwick Airport (45.6 million passengers) and Manchester Airport (27.8 million passengers). Heathrow, located west of the capital, has the most international passenger traffic of any airport in the world. It is the hub for the UK flag carrier British Airways and Virgin Atlantic. London's six commercial airports form the world's largest city airport system measured by passenger traffic with 171 million passengers in 2017. Wholesale and retail trade This sector includes the motor trade, auto repairs, personal and household goods industries. The Blue Book 2013 reports that this sector added gross value of £151.8 billion to the UK economy in 2011. As of 2016, high-street retail spending accounted for about 33% of consumer spending and 20% of GDP. Because 75% of goods bought in the United Kingdom are made overseas, the sector only accounts for 5.7% of gross value added to the British economy. Online sales account for 22% of retail spending in the UK, third highest in the world after China and South Korea, and double that of the United States. The UK grocery market is dominated by four companies: Tesco (27% market share), Sainsbury's (15.4%), Asda (14.9%) and Morrisons (10%), these supermarkets are known as the "Big Four". However discount supermarkets such as ALDI have grown in popularity. London is a major retail centre and in 2010 had the highest non-food retail sales of any city in the world, with a total spend of around £64.2 billion. Outside of London, Manchester and Birmingham are also major retail destinations, the UK is also home to many large out of town shopping centres like Meadowhall, away from the main high streets in town and city centres. Whilst the big international names dominate most towns and cities have streets or areas