subway tunnels are called pre-metro or Stadtbahn. Passengers can reach surface stations quicker than underground stations. Subjective safety at surface stations is often seen to be higher. Trams can be tourist attractions in ways buses usually aren't. Many modern tram systems plant low growing vegetation – mostly grasses – between the tracks which has a psychological effect on perceived noise levels and the benefits of greenspace. This is not possible for buses as they deviate too much from an "ideal" track in daily operations There is a well studied effect that the installation of a tram service – even if service frequency, speed and price all remain constant – leads to higher ridership and mode shift away from cars compared to buses. Conversely, the abandonment of tram service leads to measurable declines in ridership. Disadvantages Installing rails for tram tracks and overhead lines for power means a higher up-front cost than using buses which require no modifications to streets to begin operations. Tram tracks can be hazardous for cyclists, as bikes, particularly those with narrow tyres, may get their wheels caught in the track grooves. It is possible to close the grooves of the tracks on critical sections by rubber profiles that are pressed down by the wheelflanges of the passing tram but that cannot be lowered by the weight of a cyclist. If not well-maintained, however, these lose their effectiveness over time. When wet, tram tracks tend to become slippery and thus dangerous for bicycles and motorcycles, especially in traffic. In some cases, even cars can be affected. The opening of new tram and light rail systems has sometimes been accompanied by a marked increase in car accidents, as a result of drivers' unfamiliarity with the physics and geometry of trams. Though such increases may be temporary, long-term conflicts between motorists and light rail operations can be alleviated by segregating their respective rights-of-way and installing appropriate signage and warning systems. Rail transport can expose neighbouring populations to moderate levels of low-frequency noise. However, transportation planners use noise mitigation strategies to minimise these effects. Most of all, the potential for decreased private motor vehicle operations along the tram's service line because of the service provision could result in lower ambient noise levels than without. By region Trams are in a period of growth, with about 800 tram systems operating around the world, 10 or so new systems being opened each