Aghabayk et. al (2015) explained that 'T' is the driver's reaction time in GHR model (page 86). While in the article 'Traffic Dynamics. Studies in Car Following', Chandler et. al. (1958) stated that on page 181:
"... the response of the trailing vehicle depends upon its driver's perception time, t1, his response time, t2, and the time of the response of the vehicle, t3"
Which one is it, only the driver reaction time or perception+reaction+vehicle response?
Both articles are linked.
http://trid.trb.org/view.aspx?id=1342072
http://www.jstor.org/stable/167610?seq=17#page_scan_tab_contents