Traffic engineering studies how vehicles move on roads: flow, speed, queues, signals, and capacity.

Scope (traffic engineering)

This page covers macroscopic traffic flow: the link between flow, density, and speed (q = k v), capacity and level of service concepts, and the role of control (signs, signals, priority). In South Africa, highway and urban design often references SANRAL (e.g. TRH series), COLTO for roads, and municipal traffic impact guidelines. Use project-specific volume counts, design hours, and software-calibrated models for intersections—this page supports basic checks, not a traffic impact assessment sign-off.

Flow–density check (concept)

  1. Obtain or assume lane flow q (veh/h/lane) and density k (veh/km/lane) for a homogeneous segment.
  2. Verify units: q and k must refer to the same lane and length basis.
  3. Compute space mean speed from v = q/k (see calculator).
  4. For intersections, weaving, and signals, use traffic analysis software and HCM-style methods per your brief.

Transportation Engineering — Traffic Engineering

Flow, speed, density, capacity, and control—how vehicles use roads and intersections.

Introduction

Traffic engineering applies fluid-like relations on links: flow q, density k, and speed v are related by q = k v under steady conditions. Fundamental diagrams plot q–k or v–k; bottlenecks and shock waves appear when demand exceeds capacity or lanes drop. Intersections add delay and queues analysed with gap acceptance, signal timing, or microsimulation.

The calculator below solves for v given q and k; it does not replace capacity analysis or signal optimisation.

Fundamental diagram sketch showing traffic states around a bottleneck
Fundamental diagram (bottleneck / traffic states). See licence on Wikimedia Commons
Time-space diagram of vehicle trajectories on a road
Time–space diagram (vehicle trajectories). See licence on Wikimedia Commons

What traffic analysis produces

  • Capacity and level of service for links and weaving areas.
  • Delay and queue length at signalled and unsignalised intersections.
  • Design hour volumes and turning movements for geometric design.
  • Input to safety and operations studies (speed management, ITS).

Standards and guidance (South Africa)

National roads: SANRAL geometric and traffic guidance (TRH documents as cited). Provincial and municipal roads: COLTO and local by-laws. Development projects: municipal traffic impact assessment requirements. Align assumptions with the transport planning memo and counted volumes.

Steady uniform flow:   q = k v   (consistent units; often q in veh/h per lane, k in veh/km per lane, v in km/h).

Notation (common)

  • q — flow rate (veh/h per lane typical).
  • k — density (veh/km per lane).
  • v — space mean speed (km/h when q and k use the above).
  • LOS — level of service (quality of traffic conditions, method-specific).

Examples in practice

  • Freeway merge: demand vs capacity before and after auxiliary lane.
  • Urban arterial: coordinated signal timing for a corridor.
  • Roundabout: entry capacity vs circulating flow.
  • Work zone: temporary lane drop and queue warning distances.

Calculator — q = k v

Enter q (veh/h per lane) and k (veh/km per lane), k > 0. Output v = q/k in km/h.

v = q / k

Mean speed

Space mean speed v = q/k from lane flow q and density k (steady uniform flow).

Key terms

Flow (q)
Vehicles passing a point per unit time per lane.
Density (k)
Vehicles per unit length of roadway per lane.

Software and traffic analysis

Microsimulation and analytical tools model networks, signals, and priority intersections. Examples (official sites):

No product endorsement—calibrate models to local driver behaviour, heavy-vehicle share, and approved growth factors. The q–k–v calculator is a link-level spot check only.

Diagram sources

Educational schematics. Files in Images/traffic-engineering/ were downloaded from Wikimedia Commons into this repo (not copied from other topic folders). Confirm licence on each Commons file page before reuse.