Design a Toll Payment & Tracking System - System Design | WinJob

Problem Statement: Highway Toll Collection

Design Electronic Toll Collection (E-ZPass / FasTrak / SunPass-class) for highway agencies and account holders. This section covers problem statement: highway toll collection in the understanding phase.

Operational detail

Partition Kafka by gantry_id; use CP ledger per account and immutable passage facts. Edge gateways buffer 10 minutes during fiber cuts. Plate evidence lands in object storage with 90-day TTL unless dispute holds apply.

Failure and edge cases

Uplink loss with replay; cloned transponder; OCR below 0.92 confidence; DMV throttling; negative balance commuter.

Interview checkpoints

javaOne Dark Pro

1 public record TransponderId(String value, String agencyCode) { }

pythonOne Dark Pro

1 def is_low_balance(balance_cents: int, threshold: int = 500) -> bool:
2     return balance_cents < threshold

typescriptOne Dark Pro

1 export function passageId(gantryId: string, ts: number, lane: number): string {
2   return `${gantryId}:${ts}:${lane}`;
3 }

Why interviewers care

a Toll Payment & Tracking System interviews reward crisp scope, explicit trade-offs, and failure stories—not generic microservice diagrams.

Interview checkpoint

Name one failure story for Problem Statement: Highway Toll Collection that proves you understand real outages, not happy-path diagrams.

Key Highlights

•Open-road tolling (ORT) charges vehicles at highway speed without cash booths
•Dual identification: DSRC transponder primary, ALPR/LPR when tag missing or invalid
•Agency interoperability: E-ZPass IAG roaming across 19 US toll authorities
•Financial ledger must be CP; passage events tolerate brief AP at edge

1	def is_low_balance(balance_cents: int, threshold: int = 500) -> bool:
2	return balance_cents < threshold

1	export function passageId(gantryId: string, ts: number, lane: number): string {
2	return `${gantryId}:${ts}:${lane}`;
3	}