BEIJING — If you’re driving in a Chinese city in the none-too-distant future and your car is engulfed in a smooth, humming metallic belly, don’t panic. It may feel like an alien abduction, but probably it’s only a colossal, street-straddling bus.

The idea of a bus so large, high and long that it could virtually levitate above congested streets seemed surreal when presented at an expo in Beijing in May. But it came a step closer to reality this week, when a prototype went for an experimental spin in Qinhuangdao, a seaside city in northern China.

The makers of the vehicle, known as the Transit Elevated Bus, declared the ride down a few hundred yards of street on Tuesday a success, but the controlled conditions hardly reflected the gnarled unpredictability of Chinese traffic.

Television news showed the bus, resembling a goliath bug, edging forward down tracks while two cars nestled side by side underneath.