Studies have shown that the traffic flow on a two-lane road adjacent to a school can be described by the Greenshields model. A length of 0.5 mi adjacent to a school is described as a school zone (see Figure 6.19) and operates for a period of 30 min just before the start of school and just after the close of school. The posted speed limit for the school zone during its operation is 15 mi/h. Data collected at the site when the school zone is not in operation show that the jam density and mean free speed for each lane are 125 veh/mi and 57 mi/h. If the demand flow on the highway at the times of operation of the school zone is 90% of the capacity of the highway, determine: (i) The speeds of the shock waves created by the operation of the school zone (ii) The number of vehicles affected by the
school zone during this 30- minute operation
What will be an ideal response?
(i)
kj = 125, uf = 57
The traffic flow on this road section can be described by the Greenshields model
So, the capacity of this road section is given by Equation 6.25:
(ii)
In the school zone, the mean speed of traffic flow is 15 mi/h (forward),
and the shockwave speed is 4.5 mi/h (backward). Therefore, the relative speed of
the traffic flow to the end of shockwave is 15 + 4.5 = 19.5 mph.
During the 30-minute school zone operation, the affected traffic flow travels
19.5 × 30/60 = 9.75 miles.
The density of the school zone traffic is kB = 92.1 veh/mile
Therefore, the number of vehicles affected by the school zone operation is
9.75 × 92.1 = 898 vehicles
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