An ambitious light rail system runs partly underground, which gives free right-of-way and much faster traffic compared to passing the traffic signals needed in a surface level system. Underground BRT, suggested as early as 1954, is rare and expensive. As most buses run on diesel, air quality can become a significant concern in tunnels, but the Downtown Seattle Transit Tunnel is an example of using hybrid buses, which switch to overhead electric propulsion while they are underground, eliminating diesel emissions and reducing fuel usage. An alternative is an elevated busway, which is also costly. A desire for grade separation indicates that a rail alternative may be better.