Singapore, however, has used cordon charges in its current ERP scheme since 1998, when it replaced the ALS, introduced in 1975. Under the ERP, cars entering the city centre are automatically charged as they pass through gantries into the city centre during peak hours. Rates are reviewed and adjusted on a quarterly basis with the goal of maintaining speeds of 20 km/h to 30 km/h on arterial roads and 45 km/h to 65 km/h on expressways, and vary by vehicle size, entry point, day of the week and by half-hour period during peak hours.