Ambiguous speed limit after passing a T-junction

Question

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.

1. The speed limit along Fitzgerald St northbound approaching the intersection is 40 km/h (pink), due to an earlier sign.
2. The speed limit along Fitzgerald St northbound 20 metres after the intersection is 60 km/h (green), as signed explicitly.
3. 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?

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• 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:

  1. 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
  2. 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.

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Relevant parts of Road Traffic Code 2000:

Part 1 — Preliminary

3. Terms and abbreviations used

(1)
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.

Answer 1

40km/h

There is no ambiguity. The speed limit on the through road is clear and the speed limit on the side road is irrelevant.

The fact that Bob may be legitimately unaware that this is the speed limit doesn’t matter either.

If you want to ask if Bob has a defence if issued with an infringement notice, please feel free to post a new question.