Week 5 Session: The Visibility Budget

Calculating distance margins for four invented scenarios, then testing what a written scene description can and can't tell you about detection.

22 March 2027

Before the session#

Read this week’s lecture, especially the four-scenario table and the distance-margin formula — you’ll use both directly. Bring a calculator or a laptop with a spreadsheet, as in week 4. No new dataset or download is needed for this session.

The four scenarios#

This session works entirely from the same four invented, illustrative scenarios the lecture introduces — not measured effects of fatigue, glare or dusk, just labelled inputs built to isolate one variable at a time. All four use a 1,500 kg vehicle and 6.5 m/s² braking deceleration, matching week 4.

reaction distance = speed × reaction time braking distance = speed² ÷ (2 × 6.5) stopping distance = reaction distance + braking distance distance margin = detection distance − stopping distance

Two scene cards#

This session’s image exercise uses two written scene cards instead of photographs — no roadside photography exists for this unit, and a real photograph without a calibrated reference can’t establish an actual detection distance either, so a described scene serves the same discussion purpose honestly. Treat both as prompts for discussion, not as a reaction-time test, and don’t try to back-calculate a real detection distance from either description — neither is calibrated to a real distance or a real light level.

Scene card 1 — the open paddock verge. A two-lane sealed road runs straight for several hundred metres, bordered by open, grazed paddock on both sides. The light is low and orange — the sun has just gone below the horizon. A kangaroo stands side-on at the road’s edge, roughly forty metres ahead, its grey-brown coat against pale, dry grass. No oncoming traffic.

Scene card 2 — the scrub cutting. The same road enters a shallow rock cutting lined with dense roadside scrub on the passenger side. The light level is similar to scene 1. A kangaroo is partly behind a low bush, only its head and one ear visible above the scrub line, at a similar distance from the road edge. An oncoming vehicle’s headlights are visible around the next bend.

In the session#

This runs as one 50-minute sequence, after the short introductory lecture. Work in the same pairs as previous weeks.

1. 5 minutes — predict. Without calculating anything, each pair predicts which of the four scenarios has the largest distance margin, and writes one sentence saying why.
2. 10 minutes — calculate. Calculate reaction distance, braking distance, stopping distance and distance margin for all four scenarios, showing your working for at least two of them.
3. 10 minutes — isolate a variable. Pick two scenario pairs that each change only one input (for example, A→B changes only speed; B→C changes only detection distance; B→D changes only reaction time). For each pair, write one sentence stating what changed and by how much the margin moved.
4. 10 minutes — scene cards. Look at both scene cards above. For each, write two sentences: one naming a factor from the lecture (contrast, glare, vegetation, attention) that could make the kangaroo harder to detect in that scene, and one naming what would have to be true about the driver or the road for detection to happen earlier.
5. 10 minutes — redesign. Choose one scenario and change exactly one stated assumption (for example, a longer assumed detection distance, a lower speed, or a shorter reaction time). Recalculate its distance margin and write one sentence explaining what improved and why.
6. 5 minutes — submit. Write one sentence stating your overall conclusion from the four scenarios, and one sentence naming a limitation of this exercise.

Afterwards#

This session’s output is the Visibility Budget Memo — a one-page pair submission. It’s formative and ungraded: it’s practice at using the distance-margin calculation and naming its limits, not a mark on which scenario you found scariest. It does not introduce any new assessed requirement of its own.

The Visibility Budget Memo

• Your full calculations for all four scenarios (reaction distance, braking distance, stopping distance, distance margin), with units shown.
• One controlled comparison from step 3: which two scenarios, what changed, and by how much the margin moved.
• Your redesigned scenario from step 5: the changed assumption and the new margin.
• Two limitations of the exercise — for example, that an assumed detection distance isn’t a measured one, or that a positive margin doesn’t guarantee a safe outcome.

Keep your memo. Next week’s technology discussion asks where a real detection-distance number would actually have to come from, and what it would take to trust one.