Case Study — Speed Detector Reminder

TL;DR

Yahoo Carnavi — Japan's top free car navigation app — needed its first revenue stream. As main designer, I designed the Speed Detector Reminder as a paid subscription: a free-trial flow that let users feel the value before paying, validated through cognitive walkthroughs and iterative beta testing. One month after the production launch, 77.2% of trial users converted to paying users, the feature was covered by 17 media outlets, and I filed a patent as its main designer.

01The Problem

Yahoo Carnavi built its position as the top navigation service in Japan by being completely free. That strength was also its business weakness: no revenue. The company decided to introduce paid features — and market research pointed to a speed detector reminder (alerting drivers to speed enforcement points) as one of the strongest willingness-to-pay candidates.

The design challenge: introduce billing into a product whose entire brand promise was "free," without breaking user trust — and prove the value of a safety feature clearly enough that drivers would pay for it.

02My Role & Constraints

I was the main designer from the earliest stage, working closely with the PM, planner, and application engineers through design, development, testing, and release. I owned competitive research, interaction design, icon design, wireframing, prototyping, usability evaluation, and data-collection design. The billing/refund payment flow itself was owned by another team.

03Key Design Decisions

Decision 1: Let users try before they pay

For a paid safety feature, the conversion moment is everything. I designed the pre-billing experience around a free trial: an introduction flow that summarized the three core benefits in pictures and short text, so drivers could experience real alerts before being asked to subscribe. I made several versions of the introduction page, reviewed them with the design team, and converged on the final plan.

Decision 2: Make the paid value visible everywhere it matters

After subscribing, speed detectors appear on the top screen, the route-selection screen, and the navigation screen — so the value of the subscription is reinforced at every step of a drive, not buried in a settings menu. Users can also tune how long reminders are displayed, and find the introduction and refund pages from the same place.

Decision 3: Ship a beta first, then earn the production release

We launched a beta version deliberately: collected user voices at scale, prioritized them, and shipped improvements before charging in production. I designed the data collection up front so we would know — not guess — what to fix.

Prioritized improvements from beta user feedback

04Outcome

77.2%

trial → paid conversion, one month after production launch

media outlets covered the launch

9.5K RT

and 30,000 likes on the announcement tweet

As the main designer of the feature, I also filed a patent application for the Speed Detector Reminder function. Not all the feedback was positive — we kept a prioritized backlog of negative voices for the next version, because a billing feature lives or dies on retention, not launch.

05What I Learned

Charging money changes the design contract. Every ambiguity that a free user forgives, a paying user reports. Designing the trial-to-paid transition taught me that monetization UX is mostly trust UX — a lesson I carried directly into payment design at PayPay.

Designing Carnavi's first paid feature — and converting 77% of trial users into subscribers.