Drviers are usually more aggressive in large cities where traffic is very poor. But the drivers in Shanghai can still easily surprise, or even scare people if they are not prepared for the aggressive driving style.
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What makes Shanghai drivers look so aggressive? A major reason is that a lot of drivers switch lanes a lot. Taxi and bus drivers are major contribution here because they always want to drive faster so that they can get more businesses. However, the traffic of Shanghai is usually very poor and driving fast is definitely not easy. The aggressive drivers in Shanghai almost always follow the cars ahead very closely and aim at every gap in the next lane to pass the car ahead. If you are not used to this style, such a ride must look like an adventure.
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The other reason that makes them aggressive is that they are not friendly to the pedestrians crossing roads. They want the passengers to wait and let them go by first. So when they see some one going to cross the road ahead, they would only decrease the speed a little bit and keep a relatively high speed, trying to "scare" the passengers a little. of course they will still brake when they have to.
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Not everybody drives aggressively in Shanghai but there are quite some, especially among Taxi drivers. So if you are coming to Shanghai, better be parepared!