FASTag: one sticker, forty banks, and a barrier that can't wait

How India's electronic toll network works: about 120 lakh FASTag transactions a day on National Highway plazas, from 11.73 crore tags issued by 40 banks and acquired by 12. The lane decides alone in about a second, from a signed tag it can verify offline and a local exception list, and falls back to local data when the online check is slow; dozens of reads collapse into one passage with one transaction number that dedupes every later hop; the acquirer prices from the Mapper's class and presents through NPCI's switch, which settles whatever it processed, with deemed acceptance and status checks before retries; exception lists flow out as 10-minute diffs, with written liability windows (20 minutes, 60 minutes, code 176) that price their staleness; late crossings from offline plazas are processed up to 3 days, netted in three settlement cycles a day; one vehicle, one tag across every issuer; and what the Annual Pass and barrier-free tolling change.