The motor-bus is well established both in city and cross-country traffic. As at first made motor-buses consisted of special bodies with seats placed upon freight truck chassis. This did not prove altogether satisfactory because of their excessive weight, too much of which is “unsprung.” They also have a high center of gravity, high floors, long turning radius and rather rigid suspension. A bus, to be efficient, durable and comfortable, should be especially designed. There should be lightness and strength; small unsprung weight; a low center of gravity; a flexible control; special transmission; wide treads; ample wheel base; short turning radius; low step entrance and exit; low top clearance; curb receipt and delivery of passengers; ample brake capacity; and high lowgear efficiency.[167] Pneumatic tires on account of their resiliency make the bus much more comfortable for the passengers by absorbing shocks, and for the same reason they also increase the life of the car and make it possible to travel faster. Cushion tires are next in order of merit and are an effort to combine the durability of the solid tire with the easy riding qualities of the pneumatic. Tests made by the U. S. Bureau of Public Roads show that the cushion or semi-solid tires stand between the solid and the pneumatic as regards riding comfort. With many bus operators a combination equipment is being used--pneumatics are used on the front to protect the engine and gasoline tank from vibration and cushion tires on the rear where the hardest wear comes. [Illustration: A MODERN RURAL PASSENGER BUS] [Illustration: © _Underwood and Underwood_ A NEW YORK CITY “STEPLESS BUS” It Has an Emergency Door, with Wire Window Guards, and will Seat 30 Persons.] Buses are made both single and double deck. The latter are in demand where traffic is large and also where sight-seeing is an important item, the upper deck being usually open to the weather.