Electronic Road Pricing (ERP) shows up on the rate table as a wall of numbers, but the mechanism behind it is fairly simple once you break it into four parts.
1. Gantries
A gantry is the physical overhead structure you drive under. Each one has its own number (you'll see them written as "Gantry 41," for example). Driving under a gantry during a charging window is what triggers a charge — not "being in the area," a specific structure over the road.
2. Zones
LTA groups gantries that share the same rate schedule into a "zone" for publishing purposes — the CBD/Orchard cordon is the clearest example, covering dozens of individual gantries around the city core under one set of rules. Two gantries in the same zone can still be physically far apart; they just happen to charge on the same schedule.
3. Time bands
Rates step up and down through the day in short bands, usually 5-30 minutes wide, tracking how congested that stretch typically is. This is why a gantry can be free at 9:05am and charging $3 at 9:00am on the same road, on the same day. Outside of gazetted charging hours, ERP doesn't charge at all.
4. Vehicle-type multipliers
The rate table publishes one base rate per gantry per time band. What you actually pay is that base rate multiplied by a factor for your vehicle class - motorcycles pay less than cars, heavy vehicles pay more. The multiplier is fixed; only the base rate changes by gantry and time.
Where to check current numbers
Rates change a few times a year, on no fixed schedule, and this article deliberately doesn't quote specific dollar figures that would go stale. For what's charging right now, by vehicle type, see this app's ERP Rates section - or plan a trip in the Route Planner to see an estimated total for your specific route.