Ambiguous speed limit after passing a T-junction
<p>Consider the <a href="https://www.google.com/maps/@-31.9306136,115.8592416,121m/data=!3m1!1e3" rel="noreferrer">intersection between Fitzgerald St and Angove St</a> 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.</p>
<p><a href="https://i.stack.imgur.com/QsEClm.png" rel="noreferrer"><img src="https://i.stack.imgur.com/QsEClm.png" alt="Diagram of speed limit regions near the intersection" /></a></p>
<ol>
<li>The speed limit along Fitzgerald St northbound <strong>approaching the intersection</strong> is 40 km/h (pink), due to an <a href="https://www.google.com/maps/@-31.9327871,115.8591407,3a,15y,5.18h,87.13t/data=!3m6!1e1!3m4!1sPWk40C7KKvANUXpwFeM8hQ!2e0!7i16384!8i8192" rel="noreferrer">earlier sign</a>.</li>
<li>The speed limit along Fitzgerald St northbound <strong>20 metres after the intersection</strong> is 60 km/h (green), as <a href="https://www.google.com/maps/@-31.9305807,115.8591724,3a,18.3y,5.25h,84.41t/data=!3m6!1e1!3m4!1svDUGjQKlIY1EnZeH1cQAUw!2e0!7i16384!8i8192" rel="noreferrer">signed explicitly</a>.</li>
<li>The speed limit along the <a href="https://www.google.com/maps/@-31.9305423,115.8588197,3a,75y,130.69h,85.76t/data=!3m6!1e1!3m4!1s9Ohtx3fnc3eiSlf4YKxW4g!2e0!7i16384!8i8192" rel="noreferrer">Angove St approach</a> is 50 km/h (blue), the default speed limit in a built-up area.</li>
</ol>
<p><strong>What is the speed limit along the &quot;ambiguous portion&quot; (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?</strong></p>
<hr />
<ul>
<li><p>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.</p>
<p><a href="https://i.stack.imgur.com/3DeUgm.png" rel="noreferrer"><img src="https://i.stack.imgur.com/3DeUgm.png" alt="Alice going straight along Fitzgerald St northbound" /></a></p>
</li>
<li><p>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 <em>after</em> the intersection, and could conclude either:</p>
<ol>
<li>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</li>
<li>The speed limit along the ambiguous portion (yellow) is 50 km/h, since it is an unsigned portion of a built-up area.</li>
</ol>
<p><a href="https://i.stack.imgur.com/1SAPHm.png" rel="noreferrer"><img src="https://i.stack.imgur.com/1SAPHm.png" alt="Bob turning left from Angove St onto Fitzgerald St northbound" /></a></p>
<p>Either way, Bob concludes that the speed limit along the ambiguous portion is <strong>greater than</strong> 40 km/h.</p>
</li>
</ul>
<p>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.</p>
<hr />
<p>Relevant parts of <a href="https://www.legislation.wa.gov.au/legislation/statutes.nsf/law_s257.html" rel="noreferrer">Road Traffic Code 2000</a>:</p>
<blockquote>
<h2>Part 1 — Preliminary</h2>
<h3>3. Terms and abbreviations used</h3>
<p>(1)<br />
<b><i>speed zone</i></b> means a length of carriageway defined at its beginning by means of a speed limit sign, and at its end by means of —<br />
(a) a de‑restriction sign or an “end speed limit” sign; or<br />
(b) another speed limit sign; or<br />
(c) an “area speed limit” sign, a “school zone” sign or a “heavy vehicle speed zone” sign; or<br />
(d) a T‑intersection where that carriageway ends;</p>
<h2>Part 3 — Speed restrictions</h2>
<h3>11. Speed limits generally</h3>
<p>(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.<br />
Points and modified penalty: <i>see regulation 17</i>.</p>
</blockquote>

<h2>40km/h</h2>
<p>There is no ambiguity. The speed limit on the through road is clear and the speed limit on the side road is irrelevant.</p>
<p>The fact that Bob may be legitimately unaware that this is the speed limit doesn’t matter either.</p>
<p>If you want to ask if Bob has a defence if issued with an infringement notice, please feel free to post a new question.</p>

australia speeding