In London, around two-thirds of those killed in collisions involving a bus are pedestrians and most of these are killed crossing the road. The time between the pedestrian first being recognisable as a threat and the moment of impact is usually less than 2 seconds. Human drivers have very limited opportunity to avoid the collision. Automated Emergency Braking (AEB) has been developed to avoid such collisions and is becoming widespread on passenger cars. However, city buses pose a unique additional challenge. Bus operations already generate a significant quantity of non-collision injuries because passengers fall during normal operation. This includes when standing, or seated but unrestrained, passengers fall under braking. Automated brake applications where deceleration exceeds what a human driver would have applied increases this existing injury risk.

The research was sponsored by Transport for London (TfL) and aimed to quantify this balance of opportunity versus risk, and generate technical requirements allowing them to encourage or mandate AEB on their London bus fleet. The work involved:​

• Traditional collision data analysis
• Case by case review of both collision and non-collision incidents recorded by CCTV systems provided by a London bus operator
• A road trial involving an AEB-equipped bus
• AEB Performance tests on a closed test track.

Up to around 25% of bus-pedestrian fatalities could be prevented. In true positive situations, any additional risk to bus occupants was small. Human drivers rarely failed to brake in collisions with pedestrians, they just braked too late to avoid collision. Earlier intervention would mean that in some cases AEB could achieve avoidance with lower deceleration than the driver actually applied. In others, only a small increase was required.

False positives always create additional risks. The extent of the risk was strongly related to the level of deceleration and increased very substantially at 6 m/s 2 or above in the modelling. The net balance was a likely increase in slightly injured casualties but a substantial decrease in deaths and serious injuries.

Technical requirements were developed based on adaptations of the Euro NCAP standards with two false positive tests added to discourage systems that were inadequately tuned.

The analysis is strongly dependent on the rate of brake applications in service at different deceleration levels, the number of bus occupant injuries that occur at those levels and the decelerations achieved during an AEB false positive event, which is often of very short duration. Larger scale in-service trials would help to quantify these parameters more robustly.

Despite some risks, overall AEB would have strong safety potential on city buses and can be encouraged through TfL’s bus safety standard in co-operation with manufacturers and researchers to mitigate risks as far as possible.