Glossary
PLF
Piecewise Linear Function — how Optrak stores the way a stretch of road speeds up and slows down through the day, so it can work out the journey time for a vehicle leaving at any moment.
Also: Piecewise Linear Function
A piecewise linear function, or PLF, is a way of describing something that changes steadily between known points — a line graph made of straight sections joined end to end.
Optrak keeps one of these for each stretch of road, recording how long that stretch takes depending on when a vehicle sets off. The known points come from TomTom’s road-speed data, which gives a speed for every 15 minutes of the week. Joining those points up means Optrak can work out the journey time for a departure at any moment — 07:38, not just 07:30 or 07:45 — without having to store a separate figure for every minute of every road.
That is what lets a plan treat the same stretch of road as a longer journey in the morning peak than at midday, instead of using one average that is wrong at both. See congestion modelling.
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