Braking Examples

Vehicle tests to measure acceleration are more commonly available than tests of deceleration. Examples of deceleration tests below currently include only three real-world examples, e.g. road tests as opposed to track tests.

Sources: Edmonds, Car & Driver, Motor Trends, AOL.

Braking Performance

The following is a table of values from various braking tests and of values derived from them. More test figures will be added, particularly 'real-world' results as they become available. All of the current entries can be found elsewhere online. The purpose of these figures is to compare tested braking deceleration rates to those implied by the standards for yellow traffic signal timing.

In the table below:

V_i          = initial speed.
t             = time elapsed.
B_d          = braking deceleration.
G           = braking deceleration as % of g.
g            = gravitational acceleration.
Y_d          = deceleration to stop during standard yellow signal.

G/Y_d compares test deceleration to the deceleration implied by the standard for yellow signal timing. The implied deceleration is required to stop during the yellow signal if a vehicle is traveling at the speed limit at the "critical distance" from the limit line.

Note the three results from real world tests in which the deceleration rate achieved most closely approaches the deceleration implied by MUTCD yellow light timing standards.

This deceleration (.62g, see Signal Timing) would be required to stop in the following situation:

1.         A standard timed yellow signal,
2.         Vehicle traveling at the speed limit,
3.         Driver reaction time to commence braking is 1 second.
4.         Vehicle is at the distance from the limit line defined by these rates and times.

The distance in item 4 is the critical distance, as previously described.

If the car is at this critical distance and the car's braking performance as measured by G/Y_d is greater than but close to 1.00 then the required stop can be said to approach maximum performance capability of the car. If such braking performance is less than 1.00, then the required stop exceeds such maximum performance.

If the vehicle position is less than the critical distance from the limit line and the driver elects to stop, the deceleration required is higher.

At the critical distance, if the driver's reaction time and/ or decision is delayed beyond one second for any reason the deceleration required is higher.

Relatively high stopping performance in controlled track tests appears to be in the range of G/Y_d = 1.5.

A 20% reduction in deceleration time after the appearance of the yellow signal requires a G/Yd of approximately 1.55 to achieve a stop before the limit line. This is in the higher performance ranges for track tests. At 30 mph such a 20% reduction in braking time would amount to .44 seconds.

The critical distance defined here falls within a range of positions in approach to intersections previously identified as the dilemma zone. This zone is the interval of time and position in which there is reasonable intuitive uncertainty about whether to attempt to stop, or to attempt to pass through the intersection under the yellow light and perhaps part of the red.