Alliance for Lighting Information

Reaction Time and Safe Stopping Distance

by David M. Keith

Reaction time is the period between the presentation of a stimulus and the response from the subject.

In recent years different lighting researchers have reported studies of reaction time for peripheral mesopic stimuli presented during different simulations of driving. Results have consistently shown that sources with SPD's with more short wave radiation have shorter reaction times for these simulations.

The following diagram was shown in the Arizona Report 522 (and is apparently derived from work by Akashi, He, Rea and others). This figure shows that in the meso-photopic region, around 3 cd/m2, reaction times are not dependent on source type. However at lower luminances the differences in the SPD's corresponds to significant differences in reaction time. The diagram shows that to generate a reaction time around 775 milliseconds, metal halide (MH) sources should provide 0.17 cd/m2 while high pressure sodium (HPS) sources would need to provide 0.90 cd/m2 and low pressure sodium (LPS) sources around 1.30 cd/m2.

Comparisons of luminance values with the mesopic range and outdoor lighting criteria are here.

These results have been used to claim that lighting criteria for MH systems should be one-sixth of the values for HPS systems. While this may be true for those lighting systems that are designed solely to provide peripheral vision, such changes in design criteria are inappropriate for roadway and parking lot lighting and maybe walkways. Part of the reason for this is that the driving visual task is considered to have different phases, and the reaction time is only part of the total visual task. Typically, the driving visual task is considered to start with detection, followed by recognition and resposnse. The detection phase may include peripheral, para-foveal or foveal vision, depending on the location of the stimulus on the retina. The first response to detection is to look at the stimulus, and this is by definition a foveal task and needs adequate luminance for foveal vision. Therefore consideration of the driving task indicates that reducing lighting criteria because of the interaction of peripheral mesopic vision and source SPD may cause problems.

Another way to consider these results is to compare the reaction times for similar luminances. Using the 0.17 cd/m2 value corresponding to the MH reation time around 775 ms, the corresponding HPS reaction time is around 1075 ms and the LPS around 1175 ms. This is shown in the folllowing figure (note that the figure does not show the complete vertical scale down to zero.)

The difference in reaction time of 0.3 to 0.4 seconds is large compared to the 0.8 sec time for MH, but its overall significance is questionable. Such reaction time values are used in part in establishing the Safe Stopping Distance (SSD), which is defined by AASHTO. The standard reaction time for AASHTO's SSD determination is 2.5 sec. so these differences are around 15% of the total reaction time. Under standard conditions for calculating SSD, a reduction of 0.1 sec in reaction time corresponds to slowing down around 0.5 mph. Therefore under the low lighting levels shown in this diagram - 0.17 cd/m2, recommended only for e.g. local residential roadways - the difference in reaction time from MH to HPS or LPS corresponds to slowing down by 1.5 to 2 mph.

Furthermore, this analysis assumes that the headlights are off, since any contribution from headlights will increase the stimuli luminances above the recommended design criteria. With the headlights on, the luminance may be significantly increased, rising into the meso-photopic region around 1 to 3 cd/m2 where the differences in reaction time among sources are much smaller.

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