Small white bounsing light over steel bridge in calm weather.

It saw this while travelling home in a taxi over a large bridge in the city, the weather was calm, the sky was clear (nighttime), the temprature was probably around -10 to -5 centigrades. While I was in this taxi travelling westwards over the bridge(which is a large steel-structure), I saw a small white light, circular in shape heading in about 20-30 kilometers per hour in a 45 degree angle(towards north) towards the top of the bridge, I observed it moving in this fashion for about 2 seconds. I was shocked at once, thinking it could be nothing else than a helicopter crashing into the bridge. Before I had time to notify the taxi-driver, the "white light-ball" started behaving strange; when about 5 meters above the bridge it suddenly bounced straight up about 5 meters and stood there quite still for about 1 second, it was now about 10 meter above the bridge. The speed it bounced must have been huge, because I could not see it move from a(5 meters above the bridge) to b(10 meters above the bridge), it was like it just "re-positioned". Then it re-positioned in a straight line about 5 meters to the north, in the same re-positioning fashion as before, it stood there about 1/2 a second. Then it re-positioned itself 5 meters straight down, and stood there fpr about 1/2 a second, it was now almost directly above the bridge. Then it re-postioned about 8 meters to the north, then another re-position about 10-15 meters to the north, and then again about 20-25 meters to the north. These last three re-positioning happend quite fast, the light re-positioning faster for each step. Then it was gone. I recond the light could not have been much larger then about 1/2 a meter in diameter, and no smaller then 1/4 of a meter, all in white light, like a large 200 Watts light-bulb. It never changed shape, intensity or color. The taxi-driver claimed not to have seen anything.