Crash Cushion System [back]

Key Learning Points

  1. Equations of uniformly accelerated motion
  2. Newton's second law
  3. Potential energy and kinetic energy
  4. Force and momentum

Teachers' notes: (Zip 314k)

Typical results, suggested answers, students' worksheet.


In 1998-1999, there were a number of accidents happened on the highways in Hong Kong that have aroused public concern on traffic safety. One of the most serious accidents happened on 10 December, 1998 at the Tsing Ma Bridge. A speeding car rushed over a slanted divider which levitated it up to the sky. The car then flew over the bridge and crashed onto the ground at a point 20 m below the bridge. Three persons died instantly and two were seriously injured.

In 2000, the government decided to replace many slanted dividers on highways with the newly designed crash cushion systems. The new system consists of a series of boxes containing sand or water, lined up and supported by a steel framework. During a crash, the boxes burst one by one, serving as a series of cushions to absorb the energy of collision, thus minimizing the force exerted on the vehicle. By now, you can see the new system installed on many highways in Hong Kong.

This activity consists of two parts: First, you will be asked to read a newspaper article on the Tsing Ma Bridge accident and answer some questions about the accident based on your knowledge of physics. Then you will be guided to study the crash cushion system using some car test information. In the process, you will appreciate how we can apply the physics principles we have learnt to gain a better understanding of a real-life issue, and how scientists use these principles to improve road safety.

A typical crash cushion system found on a highwayThe boxes inside the system are filled with sand or water.

Part I   Newspaper Reading

Read the Mingpao newspaper article published on 11 December, 1998 about the Tsing Ma Bridge accident. Discuss the questions below with your classmates in the same group and make a brief presentation of your answers in class.

A slant concrete divider commonly found on many roads.
  1. The photo shows a typical slanted divider (±×éP) which is similar to the one involved in the accident. Describe how the divider caused the car to fly over the bridge.
  2. So why don't people use a straight concrete block, but instead they use a slanted one, to serve as the dividers on highways?
  3. In the accident, the driver was suspected to have been drunk and the car was found to be speeding. Explain, in physical terms, why these are often the major causes of traffic accidents.
  4. After the accident happened, an "expert" estimated that the car was travelling at about 200 just before the accident (as quoted in the newspaper). Later it was found that the estimate is wrong (so don't believe too much in what the so called "experts" say!) and a more reasonable estimate gives a speed of 120 . What was the corresponding kinetic energy of the car, if the car has a mass of 860 kg?
  5. It is known that the car reached a certain height in the sky and fell onto the ground under the bridge. For the sake of simplicity, assume the car fell from rest at a height of 24 m to the ground. Estimate the kinetic energy and speed of the car just before reaching the ground.

Part II   Studying the Crash Cushion System

A typical crash cushion system is shown in the photo below. You may have seen this on the road. But have you noticed that the length (or no. of boxes used) of the system depends on the speed limit of the road? We are now going to study this relation.

  1. Explain, in terms of momentum, how the crash cushion system can greatly reduce the collision force during a crash.
  2. The table below shows the result of a car test, in which the car collided with crash cushion system and was stopped in a certain distance. Notice that a car with a higher initial speed required a longer distance to stop. More boxes were thus required. The photos below show a system with six boxes installed on a road with a speed limit of 80 .

    No. of boxes

    Effective stopping distance (m)

    Initial speed of car ()













    A crash cushion system with six boxes installed on a road with speed limit 80 km h-1.

    1. Assuming the car had a uniform deceleration, write down an equation relating the initial speed, stopping distance and acceleration of the car.
    2. It was found that the cars had about the same average deceleration in the above trials. Plot a suitable straight line graph to find out the acceleration of the car.
    3. If the car had a mass of 1500 kg, and was travelling at 100 , what are the average collision force and collision time?
    4. In comparison, a car of the same mass, travelling at 100 , may stop in 0.05 s upon collision with a hard concrete barrier. Estimate the average collision force. How does this compare with the result found in (c)?

Outdoor activity

If you have the chance, take a ride on a vehicle to different highways in Hong Kong. Whenever you see a crash cushion system, record the number of boxes used and the speed limit of the road. Examine whether they are related in more or less the same way as the above data.