Tunnel protection
Published: 01 January, 2006
Current road tunnel safety is seriously limited by the traditional approach to risk assessment and intervention that are not in actual time, non-dynamic and non interactive.
Current road tunnel safety is seriously limited by the traditional approach to risk assessment and intervention that are not in actual time, non-dynamic and non interactive.
Assessment is made before or after an incident/accident and never in actual time. The assessment is based upon historical statistics and never upon actual time dynamic analysis of the holistic combination of contributing factors.
Furthermore, current risk assessment methods are separated from intervention, which is always after an incident/accident rather than before it. Crucially, the role of driver behaviour is not taken into account - and risk assessment is based upon consequences rather than causes.
Such a serious problem in tunnel safety requires a new fundamental change in approach whereby emphasis is shifted to actual time prevention of an incident/accident.
The basic technology already exists to allow this approach to be taken and individual monitoring devices are already developed in whole or in part. The new methodology could be extended to improve safety levels in applications other than road tunnels in which the public contributes to risk.
Public influence
In nuclear-chemical plants the key roles are played by the designers, management and trained operators. The general public do not directly influence safety, such that they are not included in the risk assessment.
Air and rail transport broadly fall into the same category whereby the normal public untrained 'users' play a role in self-rescue following instructions but do not influence the development towards any incident/accident.
In stark contrast, the untrained road tunnel user (vehicle driver) plays a crucial part in the development towards an incident/accident through their own 'misbehaviour'.
Surprisingly, current risk assessment and corrective actions do not consider user behaviour let alone dynamically interact with it in actual time. Reducing the negative influence of user behaviour as a preventive measure is, therefore, a critical and necessary path towards improving overall tunnel safety.
Importance of prevention
Overall tunnel safety relates to frequency of an incident occurring multiplied by the consequences, where both prevention and cure play a crucial role. Current professional practice focuses upon consequences rather than prevention. After two severe accidents with hazardous materials in chemical plants (Bhopal, Seveso) the risk analysis methodologies were adjusted to chemical plants in the late 1970s.
Quantitative risk analysis was later applied to transport first of hazardous materials and later to all transport. Risk analysis for tunnels produces a quantified risk expressed as an individual risk (e.g. fatality rate per tunnel kilometre) and a societal risk arising from the public’s aversion to high numbers of fatalities in a single accident (e.g. cumulative frequency of the total number per accident).
This is followed by risk management, which includes (a) risk evaluation comparing calculated risk with the risk acceptance criteria; (b) determination of the effectiveness of additional safety measure to reduce the risks to an “acceptable” level and (c) selection and implementation of additional safety measures.
The first aim in tunnel safety management should be to provide actual time corrective action to prevent an incident/accident occurring in the first place. In the traditional road tunnel risks assessment the main attention is directed towards mitigation and intervention after the incident/accident (rescue action, ventilation, spacing of emergency exits, etc.) to reduce the consequences of an accident.
Tunnel safety factors
Tunnel risk prior to an incident/accident varies dynamically in both time and space. It starts a distance before the entrance portal and increases at the portal after which the risk reduces depending upon physical tunnel setup, traffic conditions and user behaviour.
Physical tunnel features include tunnel length, curvature, slope, headroom, emergency lanes, lighting, signing, user-friendliness, lane widths etc. These influence traffic response such as line of vision, braking power (and temperature), engine power (and temperature), the possibilities of head-on collisions etc.
Traffic conditions include items such as traffic levels (e.g. vehicles per minute per lane) and traffic mix (e.g. light and heavy vehicles).
User behavioural influences include factors such as (a) user misbehaviour such as speeding, distance between vehicles, lane changes, overtaking and (b) psychological anxiety response to the physical and traffic tunnel conditions as well as comforting features such as user friendliness, signing, being able to see ventilation systems, emergency lanes and the tunnel exit.
Technical-management expertise worldwide, especially in Europe, is developing at such a pace as to render the new approach presented here both cost effective and feasible within a relatively short time frame in the future.
The technical requirements for monitoring of actual time traffic risk level prior to an incident rely on a combination of sensors/monitors for which the basic technology already exists - and in parts is already developed and available, including traffic speed and temperature monitoring devices. However, further development is required to allow monitoring of traffic levels/mix and distance between vehicles - as well as the development of a
computer based actual time risk assessment alarm monitor to assess the actual time risk level at individual locations and globally in time and space and present it dynamically to operators on a single monitor or even a simple analogue meter.
A further development would be to automatically control some of the safety parameters (e.g. vehicle speed, distances between vehicles) thus reducing the driver’s contribution to actual time tunnel risk.
