Thank you for your enquiry. A 'roadside test' is better described as a 'Preliminary Test'. The legal authority for such tests comes from section 6 of the Road Traffic Act 1988 (hereafter 'RTA 1988'). A preliminary test can be performed by a Police Officer in uniform who has formed a reasonable suspicion that a person is: driving; attempting to drive; or in charge of a motor vehicle and is under the influence of alcohol or drugs.
A breath test (s.6A RTA 1988) is the most commonly administered test, although there is also provision within the same legislation for an impairment test (s.6B RTA 1988) and a drug test (s.6C RTA 1988).
The breath test must only be administered at or near the place where the requirement to co-operate is made. The only exception would be where an accident has occurred (and accident is broadly defined). In those circumstances, the Police may also take you to a specified police station if they think that would be expedient. In relation to the impairment and drug test, the Police Officer in uniform can elect to carry out the procedure at or near the place the requirement is made or at a specified police station."