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Slippery when wet

July 2011


The MMDA reported a high of 30 road crashes occurring in Metro Manila. These happened during a very wet day with heavy rains brought about by a typhoon approaching Luzon. Among the high profile crashes yesterday were one involving a bus that fell from the Skyway that cost the lives of 3 people including the driver of the bus. The other involved multiple vehicles including buses (again) and several private vehicles along the underpass section of EDSA near Ayala Avenue in Makati City.

In both cases, those involved and those investigating the incidents all cited the wet and slippery road as the probably cause of the crashes. The statements were all too familiar with drivers stating that they didn’t have enough time and space to stop because of the slippery road. Such reasoning only indicates that there is the likelihood that these drivers did not apply safe driving practices to keep their distance from vehicles ahead of them and to regulate their speeds given that wet roads offer less friction or resistance that would allow for shorter braking distances.

Despite denials, it is likely that the bus that plunged from the Skyway was speeding and that the driver lost control of the vehicle. I would like to believe that the barriers of the elevated highway were designed according to standards set by the DPWH, which are based on international standards that are incorporated in their safe design manual. Despite the standard design for the barriers, the bus still managed to penetrate it, apparently elevating once it hit the barrier.

Barriers are designed according to certain test levels that are deemed suitable for the specific facility. Such test levels that need to be satisfied incorporate elements like vehicle mass, speed, angle of impact and height of center of gravity of the vehicle (Reference: DPWH Highway Safety Design Standards, Part 1: Road Safety Design Manual, 2004). Since traffic along the Skyway consists of a significant number of heavy vehicles including trucks and buses, then the barriers should satisfy the test levels appropriate for these type of vehicles. Thus, it would be easy to check if the Skyway barrier design satisfies standards by checking the as-built specifications of the facility with the DPWH manual.

Nevertheless, speeding and not keeping a safe distance between vehicles are typically causes that may be attributed to the driver. These lead to such road crashes being categorized under driver error as their cause, which is quite justified given that perhaps there are no problems pertaining to the infrastructure/road or the vehicles involved. The weather is not really to blame since it is beyond control but road conditions may deteriorate due to rains if, for example, road drainage is not properly designed or provided. In fact, hydroplaning may occur should there be significant amounts of water on a road and becomes a hazard when vehicles travel at speeds that effectively allow water to come between the tires and the pavement. In such cases, it is also necessary to check whether the tires are able to pump out water as the vehicle runs along the road. In fact, the ridges in tires are supposed to be designed to be able to pump out water, which is the basis for tires to be also checked to make sure that they are suitable for wet roads.

Basically, all drivers should heed the message from a road sign that is provided along roads that are treacherous when the pavement is wet. “Slippery when wet” actually applies to all roads whether it be of Portland cement concrete or asphalt concrete surface, whether of good or bad condition. The mere presence of water on the road pavement presents hazards that can only be addressed if drivers practice safe driving. That is, to regulate their speeds, keep safe distances and lessen maneuvers regarded as risky even when roads are dry.

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