Shamal Dust, an Unlit Lorry and 50 m Visibility
At 06:00 in March you are driving a sedan from Abu Dhabi towards Dubai on the E11. A shamal has lifted dust across the carriageway; visibility falls from 300 m to about 50 m in patches and the low sun is behind you. You are in the right-hand lane at 95 km/h. Ahead, a flat-bed lorry with no working tail lights is travelling at 60 km/h and throwing up grit that has already chipped your windscreen. Your washer bottle is half full and the wipers are smearing the glass. Traffic behind you is still moving fast and the next exit is 6 km away.
1. Rank your immediate actions over the next few seconds and justify the order.
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- Reduce speed first. Everything else can wait a second; the closing speed cannot. 95 km/h is indefensible in 50 m visibility (see the stopping-distance task).
- Lights on low beam. High beam reflects off suspended dust and returns a wall of glare.
- Clear the screen - washers and wipers together. A smeared screen beats a grit-covered one.
- Open the gap to the lorry to 4 seconds or more, reading its silhouette and its grit plume rather than tail lights it does not have.
- Do not overtake. At 50 m visibility an overtake commits you to a lane you cannot see down, and the oncoming/lane-changing risk is unquantifiable.
- Plan an exit - service area, lay-by or the exit 6 km ahead - before the dust thickens further.
The order is driven by consequence: speed reduction buys reaction time, lighting and vision buy information, and gap buys both.
2. The lorry has no rear lights. Calculate whether 95 km/h is survivable in 50 m visibility.
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You cannot stop within the distance you can see. At 95 km/h (26.4 m/s), with 1.5 s reaction and 0.7 g braking, you need about 40 m of reaction plus 50 m of braking ≈ 90 m - against 50 m of visibility.
The maximum defensible speed is the one whose total stopping distance fits inside what you can see: v × 1.5 + v²/(2 × 6.9) ≤ 50 At v = 17 m/s (about 61 km/h): 25.5 + 21 ≈ 46 m, which fits. At 18 m/s it is already about 51 m.
So the answer is roughly 60 km/h, and less if the surface is gritty - a far bigger correction than most drivers make. An unlit lorry doing 60 km/h ahead of you means the closing speed is only 35 km/h once you are at 60, and the gap you keep must be measured in seconds of travel, not metres you cannot see.
3. Visibility drops to 20 m. Is the hard shoulder a safe place to stop?
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No - it is one of the more dangerous options. In a dust storm the hard shoulder fills with vehicles stopping for exactly the same reason, and it is precisely where other drivers drift when they lose the lane. A stationary car with dim tail lights in 20 m visibility is a target, and the first vehicle to stop becomes the obstacle for the second and third.
Better options: reach the exit 6 km ahead (about 6 minutes at 60 km/h, far less at the speeds actually justified), or a service area or lay-by.
If you genuinely cannot continue: get as far off the carriageway as the ground allows, hazard lights and headlights on, occupants out on the side away from traffic and behind the barrier, and nobody standing behind or beside the car. Sitting in a stopped vehicle in the live lane, or on the shoulder, is the choice that kills people in dust and fog.