Cards markets price total bookings (often as points: yellow 10, red 25) or plain card counts. Two inputs dominate: the fixture's edge — derbies and relegation fights run hot — and the referee, whose personal cards-per-game average is a public, stable statistic most casual bettors never check.
That referee number is the closest thing to a free signal in this market. The same fixture can carry a cards expectation 30% apart depending on who holds the whistle.
A referee averaging 4.8 yellows per game takes a derby whose last five meetings averaged 5.2 cards. An over 4.5 cards line at even money prices the combination about right; the same line under a 3.1-average referee would need the derby heat to do all the work.