Now, the car decelerates at a rate $\beta $ for time $(t - {t_1})$ and finally comes to rest. Then,
$0 = v - \beta (t - {t_1})$ $⇒$ $0 = \alpha {t_1} - \beta t + \beta {t_1}$
$⇒$ ${t_1} = \frac{\beta }{{\alpha + \beta }}t$
$v = \frac{{\alpha \beta }}{{\alpha + \beta }}t$
$OA, \,AB,\, BC,\, CD$