efficiency and land space are maximized by having traffic lights on terrestrial roads, as well as the usage of interchanges such as stack interchanges. The most common forms of highway-road or highway-highway intersections are single-point urban, diamond, and trumpet interchanges. Newer expressways such as the Kallang-Paya Lebar Expressway and the future North South Corridor uses on-ramps and off-ramps to conserve space even further and minimize disruption to the road system, through the construction of viaducts and tunnels. The road surface is asphalt, unlike normal roads which may have concrete surfaces. The lanes are separated with white dashed lines, while unbroken white lines are used to mark the edges of the median and shoulder. The shoulder is reserved for stops due to breakdowns and emergencies, and motorists are prohibited by law from travelling on it. Lanes are numbered from right to left, with lane 1 being the closest to the median. Crash barriers, cat's eyes and rumble strips are also used to ensure road safety. There are signs marking the start and end of an expressway at its entry and exit points respectively. The expressways are also assigned route codes consisting three letters that form their respective initials, making the Singaporean expressway network as the only highway system with route coding system. The Expressway Monitoring and Advisory System is used on all the expressways—cameras are used for live monitoring of expressway conditions, and LED signboards display information messages, such as warnings of any disruptions to the normal flow of traffic, as well as estimated travel times. The longest expressway, the Pan Island Expressway, is only long and therefore has no rest areas. Singaporean expressways are the only highway network in Singapore with their own route codes. They are assigned with three-letter codes named after their respective initials where the last letter is always E; for example, PIE for the Pan Island Expressway. The only exception is the East Coast Parkway (ECP) whose the last letter is other than E. Safety The default speed limit and National Speed Limits on Singapore expressways is , but in certain areas a lower speed limit such as or is applied, especially in large urban areas, tunnels, heavy traffic and crosswinds. Speed traps are also deployed by the Singapore police at many places along the expressways and are deployed from 7am to 12am. Certain types of transport, such as pedestrians, bicycles, and learner drivers, are not