Signal Timing & Deceleration Requirement of MUTCD
Table 4D-102 The Manual gives the following formula for Deceleration Distance. 1. D Where: d
= deceleration rate of 10 ft/sec, (acceleration = -10 ft/sec V = either a posted or a prima facie speed limit, D t = deceleration time.
The latter two expressions of this equation are achieved by substitutions of t = V/d into Â½ dt In the Manual the deceleration distance plus reaction time
distance are added to generate a total stopping distance: 2. D = V Where t The Manual then divides by V to achieve an equation for T, the yellow light change interval as follows: 3. T = (V For present purposes t 4. T From the common expression V = at, velocity equals
acceleration times time, in the form t = V/a we see that the deceleration
yielded by dividing equation #2 above by V has doubled to 20 ft/sec a = 2d = 2(-10 ft/sec The following example illustrates. At 30 mph,
or 44 ft/sec, the time to stop (disregarding reaction distance) at a
deceleration d of 10 ft/sec V = dt, 44 ft/sec =
10 ft/sec t = 4.4 seconds. (deceleration time only considered) However the MUTCD final formula for yellow light duration, excluding reaction time, is: T = V/2d = 44 ft/sec / 2(10ft/sec In table 4D-102 (CA) the 1 second reaction time, is added producing the 3.2 second yellow interval for a 30 MPH speed limit. From this result it can be concluded that 20 ft/sec *** Some web materials on this subject seem to have mistaken the equation in the manual for the general expression relating velocity to acceleration and time. | |

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