A greater percentage of one-story administrative buildings than in urban areas. Compared to rural areas, suburbs usually have greater population density, higher standards of living, more complex road systems, more franchised stores and restaurants, and less farmland and wildlife. By 2010, suburbs increasingly gained people in racial minority groups, as many members of minority groups gained better access to education and sought more favorable living conditions compared to inner city areas. Conversely, many white Americans also moved back to city centers. Nearly all major city downtowns (such as Downtown Miami, Downtown Detroit, Downtown Philadelphia, Downtown Roanoke, or Downtown Los Angeles) are experiencing a renewal, with large population growth, residential apartment construction, and increased social, cultural, and infrastructural investments, as have suburban neighborhoods close to city centers. Better public transit, proximity to work and cultural attractions, and frustration with suburban life and gridlock have attracted young Americans to the city centers. Traffic flows Suburbs typically have longer travel times to work than traditional neighborhoods. Only the traffic within the short streets themselves is less. This is due to three factors: almost-mandatory automobile ownership due to poor suburban bus systems, longer travel distances and the hierarchy system, which is less efficient at distributing traffic than the traditional grid of streets. In the suburban system, most trips from one component to another component requires that cars enter a collector road, no matter how short or long the distance is. This is compounded by the hierarchy of streets, where entire neighborhoods and subdivisions are dependent on one or two collector roads. Because all traffic is forced onto these roads, they are often heavy with traffic all day. If a traffic crash occurs on a collector road, or if road construction inhibits the flow, then the entire road system may be rendered useless until the blockage is cleared. The traditional "grown" grid, in turn, allows for a larger number of choices and alternate routes. Suburban systems of the sprawl type are also quite inefficient for cyclists or pedestrians, as the direct route is usually not available for them either. This encourages car trips even for distances as low as several hundreds of yards or meters (which may have become up to several miles or kilometers due to the road network). Improved sprawl systems, though retaining the car detours, possess cycle paths and footpaths connecting across the arms of the sprawl system, allowing a more direct