S. Mitra, K. Utsav

Pages: 49-62

Abstract
In-vehicle headway feedback system is effective in assisting drivers in fog compared to Changeable Message Sign (CMS). In designing such feedback, it is essential to build an understanding about parameters such as safe speeds and headways in various visibility conditions, since the most critical determinant ‘timing of in-vehicle warning’ is dependent on them. Using empirical data from a fog advisory system, this study extracted data on fog driving, containing records such as time of arrivals and departures, speeds, headways and vehicle lengths and showed that a very high percentage of drivers in reduced visibility are actually in car following mode and potentially at risk of being involved in rear-end crashes. The severities of such crashes depend on the relative speeds of the leader-follower and their separation. Expressions of dynamic Potential Collision Speeds (PCS) are derived for various driving conditions based on whether the lead vehicle is stopped, decelerating or moving at a constant speed. This dynamic PCS is checked against the possible injury severity of the potential impact to issue in-vehicle Cautionary Crash Warning (CCW) or Imminent Crash Warning (ICW). A speed of 15 ft/sec has been proposed as a threshold between CCW and ICW.

Keywords: in-vehicle warning; reduced visibility; fog; car-following crash; TTC; PCS