The figure presents the time-space diagram for when the traffic on a highway is suddenly stopped for a certain time and then released. Which of the following statements are true?
- Speed is higher in Region $\mathrm{R}$ than in Region $\mathrm{P}$
- Volume is lower in Region $\mathrm{Q}$ than in Region $\mathrm{P}$
- Volume is higher in Region $\mathrm{R}$ than in Region $\mathrm{P}$
- Density is higher in Region $\mathrm{Q}$ than in Region $\mathrm{R}$