Private automobiles, whose rapid growth contributed to Bangkok's notorious traffic congestion over the past two decades, have risen in popularity, especially among tourists, expats, the upper class, and the growing middle class. A motorway network across Thailand has been gradually implemented, with motorways completed in Bangkok and most of central Thailand. Areas with navigable waterways often have boats or boat service, and many innovative means of transport exist such as tuk-tuk, vanpool, songthaew, and even elephants in rural areas.