Tyre tread depth is an important factor that helps you to assess whether your tyres are safe or do you need to change them. Tread wear indicator (TWI)  - Tyres come with tread wear indicators. You must look for a triangular arrowhead on the sidewall. You will find rubber bridges between treads parallel to the marker. If your tyre tread has worn out up to or below indicator tyres should be changed. A new car tyre comes with a tread of approximately 8mm to 9mm and the Tread wear indicators may allow you to drive it till 2mm of tread depth, however you should start looking to change the tyres once they reach a tread depth of 3mm to 4mm, because the tyres may wear out fast and you would not know when you cross this danger line. Also, once your tyres are worn out the braking distance that they could maintain goes down drastically. With new tyres you can probably stop your car from 60 kmph to 0 kmph in a braking distance of 40 feet – but with a tread depth of 3mm to 4mm this distance increases to more than 50 feet. The lower you go, say 2 mm, the braking distance becomes even more, and can go above 80 feet. So, your reaction time in circumstances of accidents or collision is much lower with worn out tyres. Be careful about tyre tread depth, and you should check it all across the tyre at least every 5,000 km or 2 months, since your tyres may look fine from one portion while the other portion may be more worn out. Decreased tyre tread depth also makes the tyres more prone to punctures.