Now when the surface is smooth , the toy car travels larger distance.
It is found that friction is caused by interlocking of irregularities on the two surfaces in contact .
Irregularities between two surfaces in contact.

The above figure shows minute irregularities on two surfaces which are locked into one another . If we want to move any surface , some part of the external force applied is used to overcome this interlocking want to maove any surface , some part of the external force applied is used to overcome this interlocking .
A rough surface has a large number of irregularities , so the force of friction is greater .
A smooth surface has small number of irregularities , so the force of friction is less.
2. Normal reaction : The force of friction depends on the normal reaction also. As the normal reaction increases , the interlocking between two surfaces in contact increases as they press harder against each other and hence friction increases .