The car started to move along a straight road.

The car starts from rest ($0 \, \text{m}$ at $0 \, \text{s}$). We observe the distance covered in successive $3 \, \text{s}$ intervals:

• From $0 \, \text{s}$ to $3 \, \text{s}$: $0.8 \, \text{m}$
• From $3 \, \text{s}$ to $6 \, \text{s}$: $2.0 - 0.8 = 1.2 \, \text{m}$
• From $6 \, \text{s}$ to $9 \, \text{s}$: $4.3 - 2.0 = 2.3 \, \text{m}$
• From $9 \, \text{s}$ to $12 \, \text{s}$: $8.0 - 4.3 = 3.7 \, \text{m}$
• From $12 \, \text{s}$ to $15 \, \text{s}$: $11.5 - 8.0 = 3.5 \, \text{m}$
• From $15 \, \text{s}$ to $18 \, \text{s}$: $14.0 - 11.5 = 2.5 \, \text{m}$
• From $18 \, \text{s}$ to $21 \, \text{s}$: $14.8 - 14.0 = 0.8 \, \text{m}$

The distance covered in each $3 \, \text{s}$ interval initially increases (from $0.8 \, \text{m}$ to $3.7 \, \text{m}$), indicating that the car is accelerating. After the $12 \, \text{s}$ mark, the distance covered in each $3 \, \text{s}$ interval decreases (from $3.5 \, \text{m}$ to $0.8 \, \text{m}$), indicating that the car is decelerating.

The motion of the car is:
The car starts from rest, accelerates for approximately the first $12 \, \text{s}$ to reach a maximum speed, and then decelerates for the remainder of the observed period.