Why Study Traffic Speed?
Speed is the most fundamental operational characteristic of a traffic stream. The speeds of different vehicles vary continuously with respect to both time (the same road at different hours) and space (different sections of the same road). Traffic engineers need standardised speed measures to capture these variations meaningfully for design, regulation, and analysis purposes.
1. Spot Speed
Spot speed is the instantaneous speed of a vehicle at a specific, defined location on the road. It is a snapshot of how fast a vehicle is travelling at one point at one moment. Spot speed is measured using instruments such as enoscopes (a simple mirror device), pressure contact tubes, loop detectors embedded in the road surface, Doppler radar guns, or laser speed meters.
Spot speed data is used in: designing horizontal and vertical curves, determining traffic signal timings, accident analysis, deciding the location and size of traffic signs, and analysing traffic capacity at a point.
2. Time Mean Speed (V_t)
Time Mean Speed is the arithmetic mean (simple average) of the spot speeds of all vehicles passing a fixed observation point during a specified time interval. It represents the speed distribution at that point over time.
V_t = ΣV_i / n
Where V_i = spot speed of the i-th vehicle, n = total number of vehicles observed.
3. Space Mean Speed (V_s)
Space Mean Speed is the harmonic mean of spot speeds taken over a road section of length L at a specific instant of time. It is obtained by dividing the total distance travelled by all vehicles by the total time they took to travel it — which effectively means slower vehicles get more weight in the average.
V_s = nL / Σt_i = n / Σ(1/V_i)
Space mean speed is the preferred measure in traffic analysis because it gives more weight to slow-moving vehicles, which is more representative of congestion and delay experienced by the average vehicle on a road section.
Relation Between Time Mean Speed and Space Mean Speed
The two speeds are related by: V_t = V_s + σ_s²/V_s
Where σ_s² = variance of space mean speed. Since σ_s²/V_s is always positive, V_t ≥ V_s always — time mean speed is never less than space mean speed.
4. Running Speed
Running speed is the average speed of a vehicle calculated over a road section, considering only the time the vehicle is actually in motion — stop delays are excluded. It reflects the actual speed during movement and is used to analyse roadway conditions.
Running Speed = Length of travel / (Journey time − Stop delays)
5. Journey Speed
Journey speed is the effective average speed of a vehicle between two points, calculated using the total elapsed time including all stops, delays, signal waits, and congestion. It reflects the overall travel experience of a road user.
Journey Speed = Length of travel / Total journey time
Journey speed ≤ Running speed always, since total journey time ≥ running time.
Percentile Speeds from Spot Speed Data
| Percentile Speed | Definition | Traffic Engineering Use |
|---|---|---|
| 15th Percentile (V₁₅) | Speed at/below which 15% of vehicles travel | Lower safe speed limit — below this causes congestion |
| 85th Percentile (V₈₅) | Speed at/below which 85% of vehicles travel | Upper safe speed limit — used for posted speed limits |
| 98th Percentile (V₉₈) | Speed at/below which 98% of vehicles travel | Design speed for highway geometric design |
Model speed is the speed at which the greatest proportion of vehicles travel — the peak of the speed frequency distribution histogram.
Key Summary
- Spot speed = instantaneous speed at a point; measured by radar, loop detector
- V_t (Time Mean Speed) = arithmetic mean of spot speeds at a point
- V_s (Space Mean Speed) = harmonic mean over section length; V_t ≥ V_s always
- Running speed excludes stops; Journey speed includes all delays
- V₁₅ = lower safe limit | V₈₅ = upper safe/speed limit | V₉₈ = design speed
- Space mean speed preferred in traffic analysis (weights slow vehicles more)