The minimum lot width shall be 100 feet at the street line for all lots; provided, however, that in the case of lots where more than 1/2 of the total frontage of the lot is on an arc or street curvature the radius of which is not more than 150 feet, the width of the lot may be not less than 55 feet at the street line and not less than 100 feet measured along the required building setback line from the street.