Design a dynamic toll collection & congestion pricing platform
PremiumBackend systemMedium
- Stream processing
- Event driven
- Financial ledger
Quick answer
This backend system interview question asks you to design a dynamic toll collection & congestion pricing platform. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design a Dynamic Toll Collection & Congestion Pricing Platform for a major metropolitan transit authority. The city has replaced manual toll booths with overhead smart gantries that automatically detect vehicles via RFID and license plate cameras.
To manage traffic flow, the city uses congestion pricing: toll amounts dynamically fluctuate based on real-time traffic volume. When a vehicle passes a gantry, the platform must determine the exact price at that specific moment, deduct the amount from the user's prepaid wallet, and notify the driver. If a vehicle is unregistered or has insufficient funds, the system must securely route the event to a violation processing queue.
What must a dynamic toll collection & congestion pricing platform do?
- Vehicle Ingestion: Receive and durably record crossing events from overhead gantries.
- Dynamic Pricing: Continuously analyze traffic volume and update the toll price for each gantry zone every 60 seconds.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Stream processing
- Event driven
- Financial ledger
- Dynamic pricing
- Idempotency
How you’ll practise
- Requirements
- Estimate
- API and data
- High-level design
- Deep dives
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any backend system design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a backend system question. You design a dynamic toll collection & congestion pricing platform and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
Related questions
Updated · Written by the SevyDevy team