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The roadside API provides information on the level of pollution very close to vehicle emission sources in busy streets and roads. The roadside API will, therefore, naturally be higher than the general API on a given day.
For most people, the roadside API will be less relevant as they spend only a short period of time each day in such locations. The roadside APIs will be of more interest to those who spend several hours continuously each day in busy streets.
(Source: EPD, HKSAR Gov't)