From a traffic study, it was observed that there are $16$ vehicles on a $320$m section of an urban road and an average time headway of $5\mathrm{.}5$ sec is observed. Calculate:$(4$ marks$)$

a$)$ Density

b$)$ Flow

c$)$ Space mean speed

d$)$ If the standard deviation of the spot speeds of the vehicles is calculated to be $20$ km$/$h$,$ calculate the time mean speed.