Why are police searching vehicles/things and mounting roadblocks?

A police officer may search such things as vehicle, ship, and an aircraft which may be conducted with or without a search warrant, depending on the nature of the thing to be searched.

Custom officers are empowered to search any vehicle, ship or aircraft reasonably suspected of carrying goods liable to forfeiture without a search warrant.[1]

A police officer may conduct a search of a thing which he reasonably believes contains anything unlawful and such a search can be conducted without a search warrant. This is derived from his general duty to prevent and detect the commission of crime[2].

The power to mount road block to stop and search, and the power to detain and search a vehicle reasonably suspected of conveying anything unlawful is derived from section 25 of the Police Act but does not cover power to extort money from the motorist.

[1] section 149 of the Customs and Excise Management Act

[2] section 4 of the Police Act