A spot speed study tells you how fast vehicles pass one point. What it cannot tell you is where they slow down and why. For that you have to travel the road with the traffic — and that is what a speed and delay study does.
This post covers speed and delay studies, the floating car method in detail, and the origin and destination studies that reveal where traffic is actually going.
Why Speed and Delay Studies Exist
We already know that a spot speed study cannot give the density of traffic, because it observes only one point.
Speed and delay studies are therefore carried out over a long distance, which makes it possible to determine the density of traffic.
What This Study Gives You
The study yields information on the amount, location and causes of delay in a traffic stream.
All three parts matter. Knowing that a journey takes twenty minutes longer than it should is of no use by itself — you need to know where those minutes are lost and what is causing the loss before you can fix anything.
This information is used in planning and in taking remedial measures to tackle delays at specific locations.
Methods of Carrying Out Speed and Delay Studies
- Floating car or riding check method
- License plate or vehicle number method
- Interview technique
- Elevated observations
- Photographic technique
The Floating Car Method
This method is suitable for two-lane traffic.
The Idea
A test vehicle is driven over a given course of travel at approximately the average speed of the stream — the driver tries to “float” with the traffic rather than lead it or lag behind it.
The name captures the technique exactly. The test car is neither fast nor slow; it drifts along with the general flow, overtaking about as many vehicles as overtake it. If that balance holds, the car is genuinely representing the average conditions.
Four observers are required in this method.
What Each Observer Does
| Observer | Records |
|---|---|
| Observer 1 | Time at various control points such as intersections, bridges and other fixed points on each trip. Also notes the amount of delay at each point. |
| Observer 2 | The time, location and cause of these delays. |
| Observer 3 | The number of vehicles overtaking the test vehicle and the number overtaken by the test vehicle. |
| Observer 4 | The number of vehicles travelling in the opposite direction on each trip. |
Observers 1 and 2 together handle delay — one measures how much, the other diagnoses why. Observers 3 and 4 handle flow — the overtaking counts reveal how well the test car matched the stream speed, and the opposing count gives the flow in the other direction.
Testing in Both Directions
The test is carried out in two directions. If the flow and average journey time of all vehicles is to be calculated in the North–South direction, then the test is run in the N–S direction as well as the S–N direction.
Both runs are needed because the count of opposing vehicles from one direction feeds into the calculation for the other.
Origin and Destination Studies
Objectives
- O & D studies determine the numbers, origins and destinations of vehicles in the zone of study.
- They provide information on the actual direction of travel, selection of route and length of trip.
- They are therefore very helpful in planning new highways and improving existing services, and are also used in planning mass rapid transit systems.
- Present and future planning of transportation systems and mass transit should be based on O & D data of passenger trips.
Six Methods of Collecting O & D Data
1. Road Side Interview Method
Vehicles are stopped at predefined interview stations and answers to prescribed questions are collected on the spot.
Information collected: place and time of origin and destination, route, location of stoppages, purpose of the trip, type of vehicle, and number of passengers in each vehicle.
| Advantage | Drawback |
|---|---|
| Data is collected quickly, in a short duration | Vehicles have to be stopped for the interview |
The drawback is more serious than it sounds. Stopping vehicles to study traffic flow interferes with the very flow being studied, and it irritates road users, which affects the quality of the answers you get.
2. License Plate Method
- The entire study area is marked out, and observers are stationed simultaneously at all points of entry and exit on every route into and out of the area.
- Each party notes the license plate numbers of vehicles entering and leaving, along with the time.
- After field data collection, the major work remaining is office computation and analysis — tracking each vehicle number and its times of entering and leaving.
Assessment: the field work is quick and easy, but it generates a great deal of office computation to trace trips through a network of stations. A large number of teams is needed to take simultaneous observations over a large area. The method is therefore most advantageous for small areas — an intersection or a small business centre.
3. Return Post Card Method
- Pre-paid business reply post cards with a return address are distributed to road users at selected points, or mailed to vehicle owners.
- A questionnaire is printed on the card, along with a request for co-operation and an explanation of the study’s purpose.
- Distribution stations are chosen where vehicles have to stop anyway, such as a toll booth.
Assessment: suitable where traffic is heavy. But only a part of the road users return the cards promptly and correctly filled. If conclusions are drawn from such incomplete returns, they may not give a true picture.
This is the classic problem of self-selection. The people who bother to return a card may differ systematically from those who do not, so the sample is biased in a way you cannot easily correct.
4. Tag on Car Method
- A pre-coded card is stuck on the vehicle as it enters the study area.
- When the vehicle leaves the marked area, the other observations are recorded on the tag.
Assessment: useful when traffic is heavy and moves continuously. Its limitation is that it gives only the points of entry and exit and the time taken to travel the area — nothing about trip purpose, route within the area, or occupancy.
5. Home Interview Method
- A random sample of 0.5 to 10 percent of the population is selected.
- Trained personnel visit their residences and collect travel data from each member of the household.
- The problem of stopping vehicles, and the difficulties that follow from it, is completely avoided.
- Care must be taken in deciding the sample size, keeping the desired accuracy in view.
This is the most thorough method. Because it asks people rather than vehicles, it captures trips made by every mode — including walking and public transport — which vehicle-based methods miss entirely.
6. Work Spot Interview Method
O & D data is collected at work spots — offices, factories, educational institutions — through personal interviews. This method plans the transportation needs of work trips specifically.
Comparing the Six Methods
| Method | Best Suited To | Main Limitation |
|---|---|---|
| Road side interview | Quick data collection | Vehicles must be stopped |
| License plate | Small areas such as an intersection | Heavy office computation; many teams needed |
| Return post card | Heavy traffic conditions | Only some cards are returned properly |
| Tag on car | Heavy, continuously moving traffic | Gives only entry, exit and travel time |
| Home interview | Comprehensive area-wide surveys | Sample size must be chosen carefully |
| Work spot interview | Work trips specifically | Covers work trips only |
Presentation of O & D Data
The data is presented in four forms:
1. O & D Table
Tables showing the number of trips between zones.
2. Desire Lines
A graphical representation prepared in almost all O & D surveys.
- Desire lines are straight lines connecting origin points with destinations.
- They show the actual desire of road users, and so help identify the need for a new road link, a diversion or a bypass.
- The width of each desire line is drawn proportional to the number of trips in both directions.
- Desire lines can be compared with the existing flow pattern along existing routes by superimposing one over the other using tracing sheets.
Why straight lines? Because a desire line shows where people want to go, not the road they are forced to take. If a thick desire line runs straight across a place where no road exists, that gap is the case for building one. Overlaying desire on actual flow is what makes the mismatch visible.
3. Pie Charts
The relative magnitude of generated traffic and the geometrical relationship of the zones involved may be shown by pie charts, in which circles are drawn with the diameter proportional to the number of trips.
4. Contour Lines
Contour lines may be plotted similar to topographic contours. The shape of the contours indicates the general traffic need of the area.
Quick Revision Notes
- Speed and delay studies cover a long distance, so they can determine density — unlike spot speed studies.
- They give the amount, location and causes of delay.
- Five methods: floating car, license plate, interview, elevated observations, photographic.
- Floating car method suits two-lane traffic and needs four observers.
- Observer 1: times and delays at control points. Observer 2: time, location and cause of delays. Observer 3: vehicles overtaking and overtaken. Observer 4: vehicles in the opposite direction.
- The test is run in both directions.
- O & D studies give numbers, origins, destinations, direction of travel, route selection and trip length.
- Six O & D methods: road side interview, license plate, return post card, tag on car, home interview, work spot interview.
- Home interview uses a sample of 0.5 to 10 percent of the population.
- Four presentation forms: O & D table, desire lines, pie charts, contour lines.
- Desire lines are straight lines from origin to destination, with width proportional to the number of trips.
Mistakes Students Commonly Make
- Saying speed and delay studies cannot give density. They can — that is the whole reason they exist.
- Getting the number of observers wrong in the floating car method. It is four.
- Mixing up Observers 1 and 2. Observer 1 records the amount of delay; Observer 2 records its cause.
- Drawing desire lines along existing roads. They are straight lines from origin to destination, regardless of the road network.
- Making desire line length proportional to trips. It is the width that is proportional.
- Forgetting that the floating car test must be run in both directions.
- Misstating the home interview sample as a fixed percentage. It ranges from 0.5 to 10 percent.
Conclusion
Speed and delay studies fill the gap that spot speed studies leave open — they travel with the traffic and so reveal where and why time is lost. Origin and destination studies go further still, showing not just how traffic behaves on the roads that exist but where people actually want to go. Desire lines are the sharpest tool in that box: draw them straight from origin to destination, overlay them on the existing network, and the case for a new link or bypass makes itself.
Frequently Asked Questions
Why are speed and delay studies carried out over a long distance?
Because observing a length of road makes it possible to determine traffic density, which a spot speed study at a single point cannot do.
What information does a speed and delay study give?
The amount, location and causes of delay in a traffic stream.
What are the methods of carrying out speed and delay studies?
The floating car or riding check method, the license plate or vehicle number method, the interview technique, elevated observations and the photographic technique.
How many observers are needed in the floating car method?
Four.
What does each observer record in the floating car method?
The first notes times and delays at control points, the second notes the time, location and cause of delays, the third counts vehicles overtaking and overtaken by the test vehicle, and the fourth counts vehicles travelling in the opposite direction.
Why is the floating car test run in both directions?
Because calculating flow and average journey time in one direction requires the count of opposing vehicles obtained from the run in the other direction.
What are the six methods of collecting O and D data?
Road side interview, license plate, return post card, tag on car, home interview and work spot interview methods.
What sample size is used in the home interview method?
A random sample of 0.5 to 10 percent of the population.
What are desire lines?
Straight lines connecting origin points with destinations, drawn with width proportional to the number of trips in both directions. They show where road users actually want to travel.
Why are desire lines useful?
Because they reveal the real travel desire of road users, which helps identify the need for a new road link, a diversion or a bypass, especially when superimposed on the existing flow pattern.
In what forms is O and D data presented?
As O and D tables, desire lines, pie charts and contour lines.