Effects of rearward countdown timers at highly automated shuttle buses to announce departing

Document Type

Journal Article

Publication Date

2026

Subject Area

place - urban, mode - bus, technology - automatic vehicle monitoring, technology - intelligent transport systems, operations - traffic, planning - safety/accidents, planning - signage/information, infrastructure - stop, infrastructure - vehicle

Keywords

Autonomous driving, Highly automated shuttle buses, External human–machine interfaces (eHMIs), Countdown timers, Situation awareness, Bus stops

Abstract

Integrating highly automated shuttle buses (HASB) into today’s road traffic promises benefits regarding traffic efficiency and accessibility. However, one challenge is the implementation of adequate interaction concepts with other traffic participants to ensure safe and comfortable encounters with HASBs. In the present study, the interaction concept to announce the HASB’s departure from a bus stop was examined with regard to the predictability of the HASB’s upcoming driving actions and the characteristics of HASB stop situations. Future potential on-demand HASB stops may require additional information provided to surrounding traffic participants by the HASB itself. The conventional interaction concept currently applied to signal departure, mainly via turn indicators, provides merely few and less detailed information. Therefore, the concept of the countdown timer (CDT) as rearward external HMI at HASBs was introduced and evaluated. CDTs provide information with high temporal precision and potentially well in advance, as HASBs usually have this information in their programmed route planning. In an experimental online study, 148 participants assessed three interaction concepts (conventional, CDT with complementing text, CDT with complementing icon) from two perspectives (following driver, pedestrian on the way to the HASB) concerning understanding/predictability, perceived interaction comfort, and acceptance. As additional behavioural measurements, participants indicated their overtaking behaviour as a following driver and behaviour when approaching the HASB as a pedestrian for each interaction concept. Results revealed for all dependent variables that participants preferred the CDT as interaction concept compared to the conventional interaction concept from both perspectives, with higher ratings for the CDT complemented with text. Behavioural measures indicated that following drivers initiate overtaking manoeuvres earlier and, therefore, faster but also choose larger and, therefore, safer time gaps when initiating overtaking manoeuvres behind a stopped HASB with a CDT. CDTs also supported pedestrians in achieving more correct assumptions regarding the reachability of the HASB. Overall, CDTs as rearward external HMIs proved to be a promising approach for safer and more comfortable interactions at HASB stops.

Rights

Permission to publish the abstract has been given by Elsevier, copyright remains with them.

Comments

Transportation Research Part F Home Page:

http://www.sciencedirect.com/science/journal/13698478

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