How do you determine the formula to count the detected vehicle?

For example if I want to count a three lane instead of two lane, how do I determine the formula for the center lane?
Example of right lane and left lane based on your code:
```
if ((abs(center.y - img.rows * 2 / 3) < 3) && img.cols / 1.55 <= center.x && center.x <= img.cols)

if ((abs(center.y - img.rows * 2 / 3) < 4) && 0 <= center.x && center.x <= img.cols / 3)
```