It is all very well to blame traffic jams, the cost of petrol and the quick pace of modem life, but manners on the roads are becoming horrible. Everybody knows that the nicest men become monsters behind the wheel. It is all very well, again to have a tiger in the bank, but to have one in the driver's seat is another matter altogether. You might tolerate the rude and inconsiderate driver, but nowadays the well-mannered motorist is the exception to the rule. Perhaps the situation calls for a 'Be Kind To Other Drivers' campaign, otherwise it may get completely out of hand. Road politeness is not only good manners, but good sense too. It takes the most cool-headed and good tempered drivers to resist the temptation to revenge then subjected to uncivilized behavior. On the other hand, a little politeness goes a long way towards relieving the tensions of driving. A friendly nod or a wave of acknowledgement in response to an act of politeness helps to create an atmosphere of goodwill and tolerance so necessary in modem traffic conditions. But such acknowledgements of politeness are all too rare today. Many drivers nowadays don't even seem able to recognize politeness when they see it. However, improper politeness can also be dangerous. Typical examples are the driver who brakes violently to allow a car to emerge from a side street at some hazard to following traffic, when a few seconds later the read would be clear anyway or the man who waves a child across a zebra crossing into the path of oncoming vehicles that may be unable to stop in time. The same goes for encouraging old ladies to cress the road wherever and whenever they want to. A veteran driver, whose manners are faultless, told me it would help if motorists learnt to filter correctly into traffic streams one at a time without causing the total blockages (堵塞)that give rise to bad temper. Unfortunately, modem motorists can't even learn to drive, let alone be well-mannered on the road. Years ago the experts warned us that the car-ownership explosion would demand a lot more give-and-take from all road users. It is high time for us to take this message to heart. According to the passage, troubles on the read are primarily caused by ______.