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Vehicle Acceleration
and Braking Parameters

-- Acceleration due to Gravity
-- Vehicle Acceleration and Braking
-- General Equations

Acceleration Due to Gravity

Objects in a free fall increase in speed with time and distance. The exact rate depends on gravity, which varies slightly with location on the earth. The earth is not a perfect sphere and gravity varies slightly depending on location. By International definition 9.80665 meters per second per second is the gravitational acceleration constant, 1g = 9.80665 m/s2 exactly.

Gravitational
Constant (1 g)

9.80665 m / s2
32.17405 ft / s2
21.93685 mph / s
35.30394 kph /s
19.06260 knots / s

ft - feet mph - miles per hour
m - meters kph - kilometers per hour
s - seconds knots - nautical miles per hour

Acceleration g's
Acceleration and deceleration (negative acceleration) is change in speed over time and can be compared to the Gravitational Constant (g). Acceleration g's is not a force, it is a fixed baseline comparing acceleration to the gravity constant. Force is mass multiplied by acceleration (F = m a).

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Vehicle Acceleration and Braking

Acceleration comparisons are for dry flat surfaces using properly inflated tires in good to excellent condition.

Fig 1.3-1

Maximum acceleration and braking depends on;

Horsepower and drive wheels are additional factors for acceleration. Production muscle cars can go from 0 to 60 mph in 5 seconds (+0.55 g's). An electric vehicle can go 0 to 60 mph in 4 seconds.

0 to 60 mph in 5 seconds
Time 0 1 2 3 4 5 sec
Speed 0 12 24 36 48 60 mph
Distance 0 9 35 79 141 220 feet


0 to 60 mph in 4 seconds
Time 0 1 2 3 4 sec
Speed 0 15 30 45 60 mph
Distance 0 11 44 99 176 feet


Calculate Acceleration
from Time and Speed
a = v / t
Speed: 0 to
Time: Seconds

Acceleration
g's

ft / s2

mph / s

m / s2

kph / s

Calculate Acceleration
from Distance and Speed
a = v2 / 2 d
Speed: 0 to
Distance:

Acceleration
g's

ft / s2

mph / s

m / s2

kph / s

Vehicle Braking
Braking rate can be expressed in acceleration g's, feet per second every second (ft/s2), miles per hour per second (mph/s), meters per second second (m/s2), or kilometers per hour per second (kph/s). Vehicle Radar Adaptive Cruise Control automatically brakes when approaching other vehicles. Braking rate varies from 0.3 to 0.5 g's.

Driver Braking
g's mph
/ sec
Description
0.30 6.6 Safe
0.35 7.7 Safe
0.47 10.3 Average Driver Max
0.50 11.0 Automatic Braking Max
0.62 13.6 Reasonably Skilled Driver Max
0.66 14.3 Skilled Driver Max
0.70 15.4 Vehicle Max
1.00 21.9 Gravity Constant
Vehicle Maximum Braking
Model g's ft/s2 mph/s
Dodge Colt GL 0.72 23.2 15.8
Chevy Blazer 0.76 24.5 16.7
Nissan Maxima 0.85 27.3 18.6
Lincoln Continental 0.92 29.6 20.2
Toyota Celica GT 0.94 30.2 20.6
BMW M3 1.0 32.3 22.0
60 mph to Stop
Production years 1991 - 1995


Convert Acceleration Units

g's
ft/s*s

m/s*s

mph/s

kph/s

knots/s

Many safety experts use 15 ft/sec2 (0.47 g's) as the maximum deceleration that is safe for the average driver to maintain control, good to excellent tires, dry surface. A reasonably skilled driver can stop at 20 ft/sec2 (0.62 g's). Most production street vehicles have a maximum braking around 0.8 g's.


General Equations


Speed and Distance
given Acceleration and Time

v = vo + a t
d = vo t + 0.5 a t2


t = time
v = speed at time t
vo = speed when t = 0
d = distance traveled at time t
a = acceleration (-a for deceleration)

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Police Radar Information Center

Acceleration Parameters
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