Public transport in Sanaa is primarily informal, with most vehicles being privately owned. Common passenger vehicles include microbuses (''dabab''s), which in 2005 were estimated to number 4-7,000 in Sanaa; minibuses ("nuss-bus"), estimated at 5,500-7,300; and taxis, which are more common at around 33,000. Public transport in the city center is well-developed, with frequently available bus and minibus services and several bus terminals (most terminals are informal), but the Sanaa outskirts are poorly served by comparison and often people "must walk long distances to reach one of the major roads." Most routes are short, meaning that longer north-south trips require switching buses multiple times. Additionally, the number of buses on the roads can vary from day to day, since bus driver licenses do not require drivers to operate on a regular schedule. This can lead to long waiting times. There are also coaches to major cities such as Aden and Taiz.