More than 9,500 breath tests and 190 drug wipes were administered during the All Wales Campaign in June and July.
Dyfed-Powys Police Assistant Chief Constable Pam Kelly said a “significant” number of motorists continued to ignore the law.
“Although this was a month-long focused campaign, the drive to change attitudes, increase road safety and catch offenders will continue year round,” she said.
Led by Dyfed-Powys Police, a total of 9,532 breath tests were administered by officers from Wales’ four police forces with 306 of those testing positive or people refused or failed to provide a test.
In Wales, England and Northern Ireland the alcohol limit for drivers is 80 milligrams of alcohol per 100 millilitres of blood, 35 micrograms of alcohol per 100 millilitres of breath or 107 milligrams of alcohol per 100 millilitres of urine
A total of 199 roadside tests for drugs were administered with 131 showing positive for drugs.
Police in England and Wales have been able to use “drugalyser” devices at the roadside since March 2015.
The mouth swab can test for cocaine and cannabis from a saliva sample within three minutes.
Drivers face prosecution if they exceed limits set for the presence of eight illegal drugs and eight prescription drugs which are sometimes abused.
Motorists who test positive can face a criminal record, loss of their licence for at least a year and a fine of up to £5,000.
The prescription drugs that fall under the new regulations are morphine, diazepam, clonazepam, flunitrazepam, lorazepam, oxazepam, temazepam and methadone.
The illegal drugs include cannabis, cocaine, ecstasy, LSD, ketamine and heroin.
Road Safety Wales chairwoman Susan Storch said: “Those that selfishly break the law are endangering themselves and, more importantly, other road users.”