There are three types of traffic signal countdown timers:
> Green Signal Countdown Timers
> Red Signal Countdown Timers
> Pedestrian Countdown Timers

Designed to reduce frustration, improve efficiency and safety, countdown timers display the time until a change in the traffic signals. However, the results are mixed in terms of safety.
Green Signal Countdown Timers can increase casualty crashes by up to 33% (Mindich, 2008) because drivers accelerate through the intersections at the end the green and tend to focus on the countdown timer rather than the activity in the intersection.
On the other hand, Red Signal Countdown Timers appear to reduce injury crashes by up to 50% and Pedestrian Countdown Timers by up to 12.5% (Srinivasan et al. 2019)

But the debate may be moot with the increasing prevalence of adaptive traffic control systems (meaning the time remaining is not known – the network sensors and algorithm decide).

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Image sources: The Outline & Chen et al.