Stopping Distance

Stopping distances of light rail depend on:

• reaction time of the driver
• maximum retardation without slip/skid based on the coefficient of friction

Reaction times of drivers of any vehicles can vary considerably. However all light rail drivers are trained, tested and professional so it can be assumed that they, and the vehicles they control, should be operating consistently to a high level of their performance capability. Occasionally drivers' reaction times may be slowed by shock, interference factors or confusion.

Shock was the probable cause of the following: 'the new Salt Lake City TRAX light-rail system suffered its first fatality. On March 30, 2000 the Salt Lake Tribune reported that after 100 days of operation, Ms. Delores Batenes, 63, was hurrying for a light-rail train in a marked crosswalk when a train she didn't see hit her. The speed of the train was estimated at 22 MPH, yet it took about 450 feet to finally stop.'

Interference with carrying out duty resulted in 25 fatalities from the Metrolink train crash on 11 September 2008 at San Fernando Valley California, 'the National Transportation Safety Board confirmed that the engineer, who died in the accident, had been text messaging while working.'

Confusion - 2 January 2005 'a Portland light-rail car smashed into and totaled a brand­ new $300,000 fire engine that was racing to someone's rescue. Emergency vehicles in Portland have "signal priority" (meaning that they automatically turn traffic signals in their favor), but the rail operator was unable to stop in time. Light rail also has signal priority, second only to emergency vehicles, so the operator probably expected the light to change to green.'

On the Gold Coast Translink says emergency vehicles have priority but it will be light rail that is given the signal priority switching system. Figure that out!

If calculating stopping distances later, don't forget to add the effect of reaction time by adding the distance the vehicle travels at the original speed before the brakes are applied. For example a vehicle travelling at 60 kph travels 17 metres during a one second reaction time.

Maximum retardation of a LRV is calculated as follows:

• The weight of the vehicle is mg where m is the mass and g is the gravitational acceleration (9.81 metres/sec/sec).
• The maximum braking force before slip occurs is calculated by multiplying the product mg by the coefficient of friction (generally no more than 0.3 for dry rail conditions).
• So the maximum retardation is 0.3 X 9.81 = 2.9 metres/sec/sec.

Note: Any car enthusiast will hope to achieve 'one g braking' - that is, 9.81 metres/sec/sec and normal truck/bus/car might expect 0.8 X 9.81 = 7.8 metres/sec/sec.

So the formula transforms to s = -u²/2a.

Assume each driver takes a one second reaction time before the brakes are applied and the stopping distances can be calculated.

The table below shows the comparative stopping distances using this calculation method plus the distance travelled during a one second reaction time:

Alternatively you can look up standard tables of stopping distances rather than calculating them for yourself.

Light rail stopping distances are markedly inferior to those of pneumatic tyred vehicles because of its low coefficient of friction and low contact area, steel wheels on steel rails.

LRV braking is further compromised in the wet and when road oil affects the steel tracks. Expect carnage from oil deliberately placed on tracks by revellers particularly during events such as Schoolies, Al SuperGP and Big Day Out.