AASHTO recommends the value of 2.5 seconds to ensure that virtually every driver will manage to react within that time. In reality, many drivers are able to hit the brake much faster. You can use the following values as a rule of thumb:
1 second – A keen and alert driver;
1.5 seconds – An average driver;
2 seconds – A tired driver or an older person; and
2.5 seconds – The worst-case scenario. It is highly probable that even elderly or intoxicated drivers will manage to react within 2.5 seconds.
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