Electronic Road Pricing (ERP), the system introduced electronic toll collection, electronic detection, and video surveillance technology. A satellite-based system was due to replace the physical gantries by 2020, but has been delayed until 2026 due to global shortages in the supply of semiconductors. As Singapore is a small island with a high population density, the number of private cars on the road is restricted with a pre-set car population quota, to curb pollution and congestion. Car buyers must pay for Additional Registration Fees (ARF) duties of either 100%, 140%, 180% or 220% of the vehicle's Open Market Value (OMV), and bid for a Singaporean Certificate of Entitlement (COE) (that varies twice a month in supply based on the number of car registrations and de-registrations), which allows the car to be driven on the road for maximum period of 10 years. Car prices are generally significantly higher in Singapore than in other English-speaking countries. As with most Commonwealth countries, vehicles on the road and people walking on the streets keep to the left (left-hand traffic). Singapore's public transport network is shaped up with trains (consisting of the MRT and LRT systems), buses and taxis. There are currently six MRT lines (North South MRT line, East West MRT line, North East MRT line, Circle MRT line, Downtown MRT line and Thomson-East Coast MRT line), three LRT lines serving the neighbourhoods of Bukit Panjang and Choa Chu Kang (Bukit Panjang LRT line), Sengkang (Sengkang LRT line) and Punggol (Punggol LRT line), and more than 300 bus routes in operation. Taxis are a popular form of transport as the fares are relatively affordable when compared to many other developed countries, whilst cars in Singapore are the most expensive to own worldwide. The Johor–Singapore Causeway (connecting Singapore with Johor Bahru, Malaysia) is the busiest international land border crossing in the world, whereby approximately 350,000 travellers cross the border checkpoints of both Woodlands Checkpoint and Sultan Iskandar Building daily (with an annual total of 128 million travellers). The Land Transport Authority (LTA) is responsible for all land transport-related infrastructure and operations in Singapore. Air Singapore is a major international transport hub in Asia, serving some of the busiest sea and air trade routes. Changi Airport is an aviation centre for Southeast Asia and a stopover on Qantas' Kangaroo Route between Sydney and London. There are two civilian airports in Singapore, Singapore Changi Airport and Seletar Airport.