More than 40,000 motorists could escape prosecution for speeding offences and a further 5,000 may wrongly have passed their driving test, the Northern Ireland Audit Office has found.
It reported that speeding drivers may not be prosecuted because the fixed penalty processing centre that deals with the offences cannot cope with the workload.
The auditor's report added that the numbers of speed cameras in Northern Ireland is "significantly lower" than that in Britain, leaving open the possibility that the speeding detection rate could be too low.
The PSNI was "unable to implement operationally" guidelines for speed enforcement issued by the Association of Chief Police Officers, it found.
The PSNI said it was aware of the guidelines on speed enforcement.
"However, it should be noted that these are guidelines to chief officers of police, they are not a requirement," it added.
"As such, each police service can, and does, set their own speed enforcement thresholds. Accordingly, PSNI, like many other police services across England, Wales and Scotland, do not apply the Association of Chief Police Officers guidelines."
The audit office further claimed that up to 5,000 new drivers could wrongly have passed their driving test. The report by the Comptroller and Auditor General found that the rate of driving-test passes dropped by some 12 per cent when it was overseen by a supervisor, meaning that there was "the potential for over 5,000 candidates to pass the test annually without meeting the required standards".
Comptroller John Dowdall admitted the North's road safety record had improved significantly. He pointed to a 31 per cent fall in the total of road fatalities and serious injuries in 2005, but added that the toll still cost the local economy more than £450 million (€666 million).
Despite the apparently better driving standards, the report points to specific areas where driver behaviour needs to improve.
This includes standards among young drivers and the rate of accidents associated with speed, alcohol and drugs.