Overview of traffic safety aspects and design in road tunnels

• Reviewing tunnels' traffic safety including the impacts of: tunnel length, fire incidents, and risk parameters.
• Short tunnels perform higher crash rates than long tunnels.
• Crash rates in tunnels are lower than in open roadways but more severe.
• Crash rates could be lower in the tunnel inner zone due to driver alertness but higher due to monotonous driving.
• Most of severe crashes occur in the in zone 4 and zone 1 (inner zone and tunnel portal).

This paper reviews aspects of traffic safety and behavior of drivers in road tunnels based on several case studies of traffic accidents along the traffic zones of tunnel alignment (entrance: zone 2; transition zone: zone 3; and inner zone: zone 4). This paper commences with engineering and design aspects that differentiate between road tunnel and open highways and, afterward, reviews certain issues related to tunnel safety and crashes such as driver behavior, highway alignment, tunnel length, and longitudinal friction. This paper additionally discusses the severity of crashes in road tunnels, specifically severe crashes in road tunnels, including fire incidents and their relationship with vehicle crashes. Finally, additional risk measures and classifications of tunnel safety are introduced.The risk of a crash in a tunnel is reduced compared with crashes on the open road (approximately half); however, tunnel crash severity is higher. The catastrophe potential related to a tunnel fire is higher than in a vehicle crash, even though fire crashes are less frequent than traffic crashes.Drivers in road tunnels generally reduce their speed and increase their lateral position from the right tunnel wall while driving. In shorter tunnels, with reduced driving speed, driver vigilance may be more robust without being hindered by dull driving, which is more common in longer tunnels. Still, in spite of driver alertness, crash rates in tunnels occur due to the tunnel's unusual driving environment. Crash rates are lower in the tunnel inner zone due to driver alertness, especially after passing the transition zone and acclimating to the tunnel environment. The number of crashes, however, is higher along zone 4 (tunnel inner zone, which is the principal zone), as it covers longer driving distance. According to most studies, short tunnels were found to exhibit higher crash rates than long tunnels because the entrance zones incorporate higher crash rates, compared with the midzones; nonetheless, longer unidirectional (freeway and multilane) tunnels with higher design speed, entail lower driver alertness and diminished concentration due to relatively monotonous driving in spite of a tunnel's closed environment.