Green Sign, Blue Sign, Quarter Tank
You are driving a hired saloon from Lagos to a 09:00 meeting in Ibadan, on a route you have never driven. At 08:05 you are on the expressway. You pass a large rectangular green sign reading 'Ibadan 45' with a route number, then a rectangular blue sign showing a fuel-pump symbol with '2 km' and a knife-and-fork symbol. Your fuel gauge sits just below the quarter mark and you have not refuelled since Lagos. The trip computer estimates a remaining range of 70 km, but it has been falling faster than the distance you have covered. A colleague in the passenger seat says the blue sign means you must leave the expressway; the driver behind is already moving into the exit lane.
1. What kind of information is each sign giving, and how should each influence your plan?
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Neither sign is an instruction.
- Green rectangular sign - directional and route information: destination and distance. It confirms you are on the right road and that Ibadan is 45 km away. Nothing about it compels or forbids a manoeuvre.
- Blue rectangular sign - services information: fuel and food are available at the next exit, 2 km ahead. It tells you a facility exists, not that you must use it.
What actually constrains the plan is your own vehicle: just under a quarter tank, an unfamiliar route, and a distance estimate that is degrading faster than reality. The signs shape the decision - they tell you where fuel is and how far the destination is - but the choice to stop is yours. Treating a blue services sign as compulsory is a misunderstanding of the sign's class: only circular signs with red borders impose obligations.
2. Using the numbers available, decide whether to take the exit. Show your margin.
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Take the exit and refuel to at least half a tank. The arithmetic:
- Distance: 45 km to Ibadan; estimated range 70 km, giving a paper margin of 25 km.
- Time: 08:05 to 09:00 is 55 minutes. At roughly 90 km/h, 45 km takes about 30 minutes, leaving around 25 minutes of slack. A ten-minute fuel stop still leaves a buffer.
- The catch: range is falling faster than distance covered, so 70 km is optimistic - real range may be nearer 55–60 km, and the margin shrinks to 10–15 km of unknown road with unknown station spacing.
A margin must be larger than the error in the estimate. A quarter tank plus an unreliable gauge plus an unfamiliar route is not a margin; it is a guess. Stopping for fuel at a known station is far cheaper than running dry on a shoulder or arriving late after a roadside wait.
3. If you take the exit, how do you use the signs to leave and rejoin safely - and why is 'blue means you must leave' wrong?
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Use the advance direction sign before the exit to get into the correct lane early, rather than cutting across at the gore. On the slip road, obey the regulatory signs at the junction (give way, stop, or directional arrows as shown), and on rejoining use the acceleration lane to match traffic speed before merging - an entry slip road is not a give-way line onto the main carriageway unless a sign says so.
The colleague's claim is wrong because sign shape and class, not colour alone, determine obligation. Rectangular signs inform and guide; circular signs with red borders prohibit or command; triangles warn. A blue services sign is information. Driver behaviour is controlled by signals, officer directions, regulatory signs and markings - not by the existence of a filling station.