Peak Hour Factor (PHF) is used to represent the proportion of peak sub-hourly traffic flow within the peak hour. If $15$-minute sub-hours are considered, the theoretically possible range of PHF will be

1. $0$ to $1.0$
2. $0.25$ to $0.75$
3. $0.25$ to $1.0$
4. $0.5$ to $1.0$