Hey Layla; here are some suggestions:

More and more people travel by car today, instead of using public transport. Why has this happened?

Nowadays (this is a very common way to start essays; it really isn't needed. It doesn't have much meaning anymore), there are more and more commuters who travel by car due to the low cost of gasoline and car itself cars themselves (to discuss count nouns in a general way, make them plural with no article). It means that people only need to pay less money ("pay" implies "money"; you don't need to write it again) to buy a car, and the. Also, the price of gasoline gasoline price is relatively lower than before. In addition, currently people are currently earning a higher pay from their work due to the burgeoning of economy. Therefore, it won’t be a big problem for them to purchase a car by themselves. Plus (not every sentence needs to start with a transition) Driving a car is more comfortable and convenient for most people, so that’s why more and more people travel by car today.

However, the extensive use of vehicles can cause ("have" sounds a bit better than "cause") a negative impact on this society. Since the roads are congested with cars today, presently people presently spend more time on the roads than before, especially during the rush hour in the city. They are not only wasting some time, but also influencing their daily work in an indirect way (how? need to explain). To make matters worse, the air pollution is ruining the city due to the car emissions. Besides, The frequent use of cars also (here is your transition word; you don't need "besides" too) makes a causes trouble for the local traffic system, which can increase the workforce ("Workforce" has another meaning) for traffic police.

In conclusion, travelling(check spelling) by car is a negative development. Since the air quality is getting worse day by day, and people may suffer a poor health in the future. And Our government may need to pay a bill for medical bills because of people’s poor health problems. I believe the extensive use of cars is not a good choice for people; instead, they need to take public transport more in order to protect the environment if possible.

thanks for sharing

-geoff