ISO 3691-4: What It Demands From Driverless Trucks

ISO 3691-4 governs every driverless industrial truck, whether it follows a wire or navigates freely. Its central demand is simple and unforgiving: the vehicle must stop within its detection field at every speed it is permitted to travel, and the field must be sized for that speed, that load and that floor. Everything else in the standard follows from that one requirement.

The core requirement, expanded

Stopping distance is the sum of four terms: the distance travelled during sensor and controller reaction, the distance travelled during brake application, the braking distance itself, and an allowance for floor condition and load. A vehicle at 1.5 m/s with a 200 ms total reaction and a 0.6 m/s² effective deceleration travels 0.3 m before braking begins and a further 1.88 m while braking, so roughly 2.2 m plus margin.

That number sets the protective field length. Reduce it and the vehicle must slow down. This is why real AMRs run far below their rated speed in narrow aisles: not caution, arithmetic.

Braking distance grows with the square of speed, so doubling from 1.0 m/s to 2.0 m/s roughly triples the required field. In a 2.5 m aisle a 4 m field is impossible, which is the geometric reason fast vehicles are slow indoors.

What else the standard requires

• Dynamic field switching. Fields must change with speed, direction and steering angle, and the switch has to be safety-rated. A vehicle turning right needs a field shaped for the swept path, not the straight-ahead one.
• Load-dependent performance. Stopping distance with a 1,000 kg pallet is not the stopping distance empty. Either the field accounts for the loaded case or the speed does.
• Manual control mode. Where an operator drives the vehicle, hazardous motion requires a hold-to-run control and reduced speed.
• Emergency stop. Accessible from the vehicle, and its actuation must not create a new hazard such as an unstable load falling.
• Warning of movement. Visual and where required audible indication that the vehicle is about to move or is moving.
• Traffic management. Where vehicles share space, the system must prevent collisions between vehicles, which is a system-level requirement rather than a vehicle one.
• Zones and doors. Interfaces to lifts, doors and shutters need interlocking so a vehicle cannot drive into a closing door or an absent lift car.

The requirements integrators miss

1. Fields sized for an empty vehicle. Commissioning happens without load, and the loaded stopping distance is never re-measured.
2. Reverse travel unprotected. Many vehicles have no rear scanner and rely on speed restriction. If the speed restriction is not safety-rated, the protection does not exist.
3. Ramps. Gradients change braking distance substantially, and few field configurations account for them.
4. Floor condition. A wet or dusty floor reduces effective deceleration. The standard expects the foreseeable worst case, not the showroom case.
5. Vehicle-to-vehicle collisions. Scanners detect people; two vehicles approaching at a blind corner is a traffic management problem that software must solve.
6. Overhanging loads. A pallet wider than the vehicle extends the swept path beyond the protective field.

Legal frame in Europe

ISO 3691-4 is the harmonised technical route to compliance, while the legal obligation comes from machinery legislation. Directive 2006/42/EC applies to machinery placed on the European market until 19 January 2027, and Regulation (EU) 2023/1230 applies from 20 January 2027, with no overlapping period. Fleets being specified now for 2027 delivery should be assessed against the regulation.

Frequently asked questions

Sources