Since Singapore is a small island with a high population density, the number of private cars on the road is restricted so as to curb pollution and congestion. Car buyers must pay for duties one-and-a-half times the vehicle's market value, and bid for a Singaporean Certificate of Entitlement (COE), which allows the car to run on the road for a decade. The cost of the Singaporean certificate of entitlement alone would buy a Porsche Boxster in the United States. Car prices are generally significantly higher in Singapore than in other English-speaking countries.[250] As with most Commonwealth countries, vehicles on the road and people walking on the streets keep to the left.