A single-lane highway has a traffic density of $40 \; \text{vehicle/km}.$ If the time-mean speed and space-mean speed are $40 \; \text{kmph}$ and $30 \; \text{kmph},$ respectively, the average headway $\text{(in seconds)}$ between the vehicles is

1. $3.00$
2. $2.25$
3. $8.33 \times 10^{-4}$
4. $6.25 \times 10^{-4}$