Compared with cars, bikes have many advantages(这应该是主题句，在这么短的文章里应该放在第一段). First, they are not very expensive, and almost every family can afford them. (这句话分成2个句子比较好，一句是key point,一句做进一步解释)Second, they are very handy and convenient though they are not so fast as cars. With a bike, you can go anywhere you like, and you don't have to look for a large parking place. By riding bikes, you can avoid traffic jams and thus save the time and the money for taking taxis(同第一点一样，但这一点思路有些乱). Third, they do not cause air or sound pollution(没有解释，这一点肯定拿不到好分数).
Riding bikes does good to your health if you ride it regularly(这应该是文章要论述的第四点，但没有连接词，又分了段，打乱了文章的整体结构). In modern times, people are usually too busy to spare any time for physical exercise. Riding a bike to your work place regularly serves as a good way to keep yourself strong and healthy. (2个句子的顺序换一下比较好。因为前面已经说了有益于健康，读者等着听解释，但作者却换了话题。第一句话应该是第二句话的原因。) Therefore, a bicycle is really an important means of transportation in China. It has been very useful, convenient and necessary ever since it was invented. And I do believe it will still be so in the future.(结论态度不鲜明，没有紧扣主题句。 应该重述四个优点，然后给出结论)
