Consider the intersection between Fitzgerald St and Angove St in North Perth, Western Australia (left-hand traffic). This is a T-junction, with Fitzgerald St being the continuing road and Angove St being the terminating road.
- The speed limit along Fitzgerald St northbound approaching the intersection is 40 km/h (pink), due to an earlier sign.
- The speed limit along Fitzgerald St northbound 20 metres after the intersection is 60 km/h (green), as signed explicitly.
- The speed limit along the Angove St approach is 50 km/h (blue), the default speed limit in a built-up area.
What is the speed limit along the "ambiguous portion" (yellow) of Fitzgerald St northbound, that is, the 30-metre portion of Fitzgerald St from the left-turning lane of Angove St up to the 60-sign?
Suppose Alice travels on Fitzgerald St northbound, and drives straight through the intersection. Alice would conclude that the speed limit along the ambiguous portion (yellow) is 40 km/h, since the earlier 40-sign on Fitzgerald St applies all the way up to the 60-sign.
Suppose Bob approaches on Angove St and turns left into Fitzgerald St northbound. Bob knows nothing about the earlier 40-sign, and only ever sees the 60-sign on Fitzgerald St after the intersection, and could conclude either:
- The speed limit along the ambiguous portion (yellow) is 60 km/h, since the 60-sign is consistent with all of Fitzgerald St being a 60-zone (which was indeed the case before 2016); or
- The speed limit along the ambiguous portion (yellow) is 50 km/h, since it is an unsigned portion of a built-up area.
Either way, Bob concludes that the speed limit along the ambiguous portion is greater than 40 km/h.
In practice it is unlikely that Bob, after completing the left turn, could actually manage to accelerate to a speed greater than 40 km/h within the 20-metre ambiguous portion up to the 60-sign. But it seems that the speed limit in the ambiguous portion is ill-defined, meaning that Alice (going straight ahead) might be able to get away with doing 50 km/h or 60 km/h.
Relevant parts of Road Traffic Code 2000:
Part 1 — Preliminary
3. Terms and abbreviations used
speed zone means a length of carriageway defined at its beginning by means of a speed limit sign, and at its end by means of —
(a) a de‑restriction sign or an “end speed limit” sign; or
(b) another speed limit sign; or
(c) an “area speed limit” sign, a “school zone” sign or a “heavy vehicle speed zone” sign; or
(d) a T‑intersection where that carriageway ends;
Part 3 — Speed restrictions
11. Speed limits generally
(2) A person must not drive a vehicle in a built‑up area, at a speed exceeding 50 km/h, except within a speed zone in which a higher speed is permitted.
Points and modified penalty: see regulation 17.