Short note: traffic flow parameters

Traffic flow is described by three basic macroscopic parameters, which are related to each other, and several microscopic ones.

1. Flow or volume ($q$): the number of vehicles passing a section per unit time, in veh/h or PCU/h. Related measures: AADT, peak hour volume, design hourly volume, PHF and rate of flow.

2. Speed ($v$): the distance covered per unit time.

• Spot speed: at a point.
• Time mean speed: average of spot speeds, $\bar v_t = \frac{1}{n}\sum v_i$.
• Space mean speed: average over a length, $\bar v_s = \frac{n}{\sum 1/v_i} = L/\bar t$.
• Running speed (excluding stops) and journey speed (including stops).

3. Density ($k$): the number of vehicles per unit length of road, in veh/km. It is related to occupancy.

Fundamental relation

$$q = k\,\bar v_s$$

With a linear model $v = v_f(1 - k/k_j)$ the capacity is $q_{max} = v_f k_j/4$ at $k = k_j/2$.

4. Time headway ($h$) and space headway or spacing ($s$): $h = 3600/q$ and $s = 1000/k$; they describe how vehicles are spaced in the stream.

5. Other parameters: capacity, level of service, v/c ratio, gap, clearance, platoon size and composition.

These parameters are measured by traffic surveys (volume count, spot speed, floating car) and used for design of roads and signals.