Traffic Flow Fundamental

q = k v, spacing s = 1/ k

Flow q (veh/h) equals density k (veh/mi or veh/km) times space-mean speed v. The fundamental diagram is typically triangular or parabolic with maximum flow at capacity near ½ k_j in the Greenshields model. Headway h = 1/q.

$q$ (veh/h): Flow rate
$k$ (veh/mi): Density
$v$ (mph): Space-mean speed

Step-by-step solved example

A freeway lane has 22 veh/mi traveling at 55 mph space-mean speed. Flow?

q = k v. Vehicles occupy space along the lane.

1. q = kv
q = 22 × 55 = 1210 veh/h/ln.
$q = 1210 veh/h$

Answer: 1210 veh/h/lane

10 practice questions

0/10 correct

1.Time mean speed is

2.If average headway is 2.0 s, q is

3.Jam density k_j is density when

4.Greenshields: capacity occurs at

5.AADT is

6.PHF (peak hour factor) is typically

7.v = 40 mph, k = 40 veh/mi. q =

8.HCM LOS is based on

9.A 15-min count of 400 veh on a lane: hourly flow rate =

10.Space-mean speed from two travel times over a fixed length is