The outback is criss-crossed by historic tracks. Most of the major highways have an excellent bitumen surface and other major roads are usually well-maintained dirt roads. Tracks in very sandy or exceedingly rocky areas may require high-clearance four wheel drives and spare fuel, tyres, food and water before attempting to travel them, however most outback roads are easily traversed in ordinary vehicles, provided care is taken. Drivers unused to dirt roads should be especially cautious – it is recommended that drivers reduce their speed, drive with extra care, and avoid driving at night because animals can stray onto roads. Travelling in remote areas in northern Australia is not advisable during the wet season (November to April), as heavy tropical downpours can quickly make dirt roads impassable. In the remotest parts of Australia fuel sellers are located hundreds of kilometres apart, so spare fuel must be carried or refuelling spots calculated carefully in order not to run out of fuel in between towns. In addition, multiple trailer trucks (known as Road Trains) traverse these roads and extreme care must be taken when around these vehicles, due to their weight, length (often three full trailers long) and amount of dust thrown up by over 46 tyres.