Ultra is an on demand system of driverless automatic taxis traveling on their own guideway network. It's developed at the University of Bristol, UK. Its guideway has the same capacity as a motorway lane but whereas the average speeds in city centers are 13 km/h (8 mph) the Ultra speed is 40 km/h, three times faster. The Ultra uses much less energy per passenger km than buses, cars, trains or light rail and is environmentally friendly, zero city pollution. The Ultra also travels non-stop to the destination, transporting 4 passengers in comfort. One of every two people live in cities and the immigration to cities is increasing, creating megalopolises of millions to tens of millions inhabitants. More than 200 cities have a population of more than a million people. Managing such large cities and their huge traffic jams and better management of the planet’s resources, could become the most difficult problem of the next century. The Ultra system costs less to operate than buses, cars, trains or light rail, and is typically only one third the cost of equivalent light rail to install.