All Red time
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Help Support Professional Engineer & PE Exam Forum:
First you look up the standard yellow clearence intervals (based on approach speed, from 3s-5s)
Then calculate the non-dilema change period (yellow + all -red)
CP = t + V/2a + (W+L)/V
t = PRT (taken as 1 s)
V = approach speed (ft/s)
a = decelleration rate ft/s
W = width of intersection
L = Length of vehicle (in an example, L was a standard 20 ft)
To calculate the all-red, subtract the standard yellow clearence interval from the non-dilema change period.
My reference was ITE's 'manual of signal design' but there is some stuff in HCM and the GreenBook too.
Then calculate the non-dilema change period (yellow + all -red)
CP = t + V/2a + (W+L)/V
t = PRT (taken as 1 s)
V = approach speed (ft/s)
a = decelleration rate ft/s
W = width of intersection
L = Length of vehicle (in an example, L was a standard 20 ft)
To calculate the all-red, subtract the standard yellow clearence interval from the non-dilema change period.
My reference was ITE's 'manual of signal design' but there is some stuff in HCM and the GreenBook too.
My Principles of Highway Engineering have it simply as:
AR = (w+l) / V
As noted, it is also good to be aware that:
Y= tr + V / (2a + 2gG)
And Change and Clearance Interval = AR + Y
AR=All Red, usually rounded up to the nearest .5 sec
w=width of cross street
l=vehicle length, usually 20 ft
tr=driver perception/reaction time - usually taken as 1 sec
V in ft/s
Y=yellow time, usually rounded up to the nearest .5 sec. Typically in the range of 3 to 5 seconds.
a=deceleration, usually taken as 10 ft/s^2
g=32.2 ft/s^2
G=% Grade divided by 100
AR = (w+l) / V
As noted, it is also good to be aware that:
Y= tr + V / (2a + 2gG)
And Change and Clearance Interval = AR + Y
AR=All Red, usually rounded up to the nearest .5 sec
w=width of cross street
l=vehicle length, usually 20 ft
tr=driver perception/reaction time - usually taken as 1 sec
V in ft/s
Y=yellow time, usually rounded up to the nearest .5 sec. Typically in the range of 3 to 5 seconds.
a=deceleration, usually taken as 10 ft/s^2
g=32.2 ft/s^2
G=% Grade divided by 100
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PsychoNumber1
Awesome
it's funny that there are equations for this, considering there are jurisdictions out there that use 0s or 0.5s all red time as a blanket application. :screwloose:
I also find this topic interesting and amusing, since I work in Washington, DC, with the 3rd worst traffic in the country. There are lights with a green time of 9 seconds; some are 90 seconds or longer... For some of them, during rush, they are always green, yet there is still a backup. Fun, fun, fun....
