Volume tells you how many vehicles are using a road. Capacity tells you how many could. And level of service tells you something neither of those numbers captures — what the journey actually feels like.
This post covers the three types of capacity, the capacity formulas, the six levels of service, passenger car units, and the IRC capacity values. It is the longest topic in the chapter and one of the most heavily examined.
Capacity, Volume and Density
Three related quantities have to be kept apart.
| Quantity | Definition | Unit |
|---|---|---|
| Traffic volume | Number of vehicles moving in a specified direction on a given lane or roadway that pass a point during a specified unit of time | vehicles per hour or per day |
| Traffic density | Number of vehicles occupying a unit length of lane at a given instant | vehicles per kilometre |
| Traffic capacity | The ability of a roadway to accommodate traffic volume — the maximum number of vehicles that can pass a given point in unit time | vehicles per hour per lane |
Traffic volume = traffic density × traffic speed
This relation contains a small paradox worth noticing. The highest density occurs when vehicles are practically at a standstill — packed bumper to bumper. But at a standstill the speed is zero, so the volume approaches zero. Maximum density and maximum volume are not the same thing at all; a jammed road carries almost nothing past a given point.
Capacity versus Volume
Capacity and volume are both measures of traffic flow and share the same units, but they mean different things:
- Volume represents an actual rate of flow and responds to variations in traffic demand.
- Capacity indicates a capability — the maximum rate of flow, at a certain level of service, that the roadway can carry.
Capacity depends on the prevailing roadway and traffic conditions.
The Three Types of Capacity
1. Basic Capacity
Basic capacity is the maximum number of passenger cars that can pass a given point on a lane or roadway during one hour under the most nearly ideal roadway and traffic conditions that can possibly be attained.
- Two roads with the same physical features have the same basic capacity, regardless of traffic conditions — because conditions are assumed to be ideal in both cases.
- Basic capacity is therefore the theoretical capacity.
2. Possible Capacity
Possible capacity is the maximum number of vehicles that can pass a given point on a lane or roadway during one hour under prevailing roadway and traffic conditions.
- It is generally much lower than basic capacity, because real conditions are seldom ideal.
- In the worst case, when congestion brings traffic to a standstill, the possible capacity may approach zero.
- As conditions approach the ideal, possible capacity approaches basic capacity.
Possible capacity varies from zero to basic capacity.
3. Practical Capacity (Design Capacity)
Neither basic nor possible capacity can be used for design, since they represent the two extreme cases — the perfect and the disastrous.
Practical capacity is the maximum number of vehicles that can pass a given point on a lane or roadway during one hour without traffic density becoming so great as to cause unreasonable delay, hazard, or restriction to the driver’s freedom to manoeuvre, under prevailing roadway and traffic conditions.
This is the capacity of primary interest to designers, who need to provide adequate highway facilities. For that reason it is also called design capacity.
The Three Compared
| Type | Conditions Assumed | Relative Value | Used For |
|---|---|---|---|
| Basic | Ideal | Highest | Theoretical reference |
| Possible | Prevailing | Zero up to basic | Describing actual conditions |
| Practical | Prevailing, without unreasonable delay | Between the two extremes | Design |
Determining Theoretical Maximum Capacity
The theoretical maximum or basic capacity of a single lane follows from a simple idea: if you know how fast vehicles travel and how much space each one occupies, you can work out how many pass per hour.
C = 1000 V / S
| Symbol | Meaning | Unit |
|---|---|---|
| C | Capacity of a single lane | vehicles per hour |
| V | Speed | km/hr |
| S | Average centre-to-centre spacing of vehicles when following one behind another in a queue (space headway) | m |
The 1000 converts kilometres to metres, so that V in km/hr divided by S in metres gives vehicles per hour.
Building Up the Spacing
The minimum space gap — the clear gap between vehicles — is the distance covered during the reaction time:
Sg = 0.278 V t m (V in km/hr, t in seconds)
The 0.278 is simply the conversion from km/hr to m/s, being 1/3.6.
The minimum space headway adds the length of the vehicle itself, since spacing is measured centre to centre:
S = Sg + L = 0.278 V t + L
An empirical relation for spacing is also used:
S = 0.2 V + L m
Capacity from Time Headway
Approaching the same problem through time rather than distance:
C = 3600 / Ht
where C is capacity in vehicles per hour and Ht is the minimum time headway in seconds. If vehicles pass every 2 seconds, then 3600/2 = 1,800 vehicles pass in an hour. That is all the formula says.
Six Factors Affecting Practical Capacity
1. Lane Width
As lane width decreases, capacity decreases. The practical capacity of a 3.0 m wide lane on a two-lane rural road may fall to 76 percent of the capacity of a 3.5 m lane.
Losing half a metre of width costs nearly a quarter of the capacity — a striking return on a small dimension.
2. Lateral Clearance
Vertical obstructions such as retaining walls or parked vehicles near the traffic lane reduce the effective width of the lane and so reduce capacity.
- A minimum clearance of 1.85 m from the pavement edge to the obstruction is considered desirable.
- When the clearance falls to 0.75 m on one side only, capacity drops to about 96 percent.
- When the obstruction is on both sides, capacity falls further to 80 percent of the standard design capacity.
Note that the obstruction does not physically narrow the pavement at all. Drivers simply shy away from it, and the road behaves as though it were narrower.
3. Width of Shoulders
Narrow shoulders reduce the effective width of the traffic lanes, because vehicles travel towards the centre of the pavement when they feel there is nowhere to go on their outer side.
4. Commercial Vehicles
Large commercial vehicles such as trucks and buses occupy greater space and influence other traffic in the same lane as well as vehicles in adjoining lanes.
5. Alignment
Restrictions to sight distance reduce capacity, and steep and long grades affect it too — a slow, heavily laden truck on a long climb holds back everything behind it.
6. Presence of Intersections at Grade
Intersections restrict the free flow of traffic and adversely affect capacity. The capacity of an intersection of two roads crossing at grade is slightly less than the lower capacity of the two roads.
Level of Service
Capacity is a number. But two roads carrying identical volumes can feel completely different to drive on — one flowing freely, the other a grinding queue. Level of service captures that difference.
Capacity is a quantitative measure. Level of service is a qualitative measure of flow.
Level of service is defined as a qualitative measure describing operational conditions within a traffic stream and how they are perceived by drivers and passengers.
- Capacity standards are normally fixed in relation to the level of service.
- Capacity depends on geometric design, facilities, environmental conditions and so on.
- Six levels of service are recognised, designated A to F. Level A is the best operating condition (free flow) and Level F is the worst (forced or breakdown flow).
Level of Service A — Free Flow
- Average travel speeds about 90 percent of free-flow speed for the arterial class.
- Individual users are virtually unaffected by the presence of others.
- Freedom to select desired speed and to manoeuvre is high.
- Comfort and convenience are excellent.
Level of Service B — Stable Flow
- Average travel speeds about 70 percent of free-flow speed.
- A zone of stable flow, with drivers still having reasonable freedom to select their speed and manoeuvre.
- Comfort is somewhat less than Level A, because the presence of other vehicles begins to affect individual behaviour.
Level of Service C — Stable Flow, Interaction Begins
- Average travel speeds about 50 percent of free-flow speed.
- Still stable flow, but it marks the beginning of the range where individual users become significantly affected by interactions with others.
- Speed selection is affected by others, and manoeuvring requires substantial vigilance.
- Comfort and convenience decline noticeably.
Level of Service D — Approaching Unstable Flow
- Average travel speeds about 40 percent of free-flow speed.
- Represents the limit of stable flow, with conditions approaching unstable flow.
- High density severely restricts freedom to select speed and manoeuvre.
- Comfort and convenience are poor.
- Small increases in flow usually cause operational problems.
Level of Service E — At Capacity
- Average speed about one-third of free-flow speed.
- Traffic volumes are at or close to the capacity level.
- Speeds are reduced to low but relatively uniform values.
- Manoeuvring is extremely difficult, generally achieved by forcing another vehicle to give way.
- Comfort and convenience are extremely poor and driver frustration is high.
- Operations are unstable — small increases in flow or minor disturbances cause breakdowns.
Level of Service F — Forced or Breakdown Flow
- Average travel speeds between 25 and 33 percent of free-flow speed.
- Occurs when the traffic approaching a point exceeds the amount that can pass it.
- Queues form behind such locations.
- Operations within the queue are characterised by stop-and-go waves, which are extremely unstable.
- Vehicles may move at reasonable speed for several hundred metres and then have to stop, in a cyclic fashion.
- High volumes cause breakdown, with long queues and delays.
All Six at a Glance
| LOS | Speed as % of Free Flow | Flow Condition |
|---|---|---|
| A | About 90 % | Free flow |
| B | About 70 % | Stable flow |
| C | About 50 % | Stable flow, interactions significant |
| D | About 40 % | Limit of stable flow |
| E | About 33 % (one-third) | At capacity, unstable |
| F | 25 % to 33 % | Forced or breakdown flow |
The speed sequence 90, 70, 50, 40, 33 is worth committing to memory as a chain — it is a very common objective question.
Passenger Car Unit (PCU)
Indian traffic contains cars, trucks, scooters, cycle rickshaws and bullock carts sharing the same lane. Counting them all as one “vehicle” each would be meaningless, since a bullock cart and a scooter affect traffic in utterly different ways.
It is difficult to estimate volume and capacity under mixed traffic unless the different vehicle classes are converted to one common standard vehicle unit. The passenger car is taken as that standard, and the unit is called the Passenger Car Unit (PCU).
- In mixed traffic, volume and capacity are expressed as PCU per hour or PCU per lane per hour, and density as PCU per kilometre of lane.
- If one vehicle of a class produces the same effect as one passenger car, that class has a PCU value of 1.0.
- The PCU value of a class may be considered as the ratio of the capacity of a roadway carrying passenger cars only, to its capacity when carrying only vehicles of that class.
Factors Affecting PCU Values
- Speed of the vehicle under the prevailing roadway and traffic conditions, within the desired speed range
- Length and width of the vehicle
- Transverse and longitudinal gaps allowed between vehicles of the same class in the speed range under consideration, during compact stream flow
IRC Equivalency Factors — Rural Roads in Plain Terrain
| S. No. | Vehicle Class | Equivalency Factor |
|---|---|---|
| 1 | Passenger car, tempo, auto-rickshaw, agricultural tractor | 1.0 |
| 2 | Bus, truck, agricultural tractor-trailer unit | 3.0 |
| 3 | Motor cycle, scooter, pedal cycle | 0.5 |
| 4 | Cycle rickshaw | 1.5 |
| 5 | Horse drawn vehicles | 4.0 |
| 6 | Small bullock cart and hand cart | 6.0 |
| 7 | Large bullock cart | 8.0 |
A large bullock cart counts as eight passenger cars. That is not about size — it is about speed. A very slow vehicle in a faster stream disrupts traffic far out of proportion to the space it occupies, because every following vehicle must slow, wait and eventually overtake.
IRC Practical Capacity Values for Rural Roads
| Type of Road | Capacity, PCU per Day (Both Directions) |
|---|---|
| Single lane, 3.75 m carriageway with normal earthen shoulders | 1,000 |
| Single lane, 3.75 m carriageway with 1.0 m hard shoulders | 2,500 |
| Intermediate lane, 5.5 m width with normal earthen shoulders | 5,000 |
| Two lane, 7.0 m carriageway with earthen shoulders | 10,000 |
| Four lane divided highway (depending on traffic, access control, etc.) | 20,000 to 30,000 |
Look at the first two rows. Same carriageway width, but adding 1.0 m of hard shoulder raises capacity from 1,000 to 2,500 PCU per day — a 150 percent increase from a metre of surfaced strip that carries no traffic at all. Hard shoulders let vehicles pass one another safely, which is exactly what a single lane road cannot otherwise do.
Parking Studies
Parking space is one of the major problems of highway transportation. Three aspects are investigated:
1. Parking Demand
Evaluated by different methods, including making a cordon count of a selected area and recording the accumulation of vehicles during peak hours, by subtracting the outgoing traffic from the traffic volume entering the cordoned area.
2. Parking Characteristics
Analysing the present parking facilities and finding the general problems — including accidents during parking and unparking operations.
3. Parking Space Inventory
The area under study is fully surveyed and a map is prepared showing all places where kerb parking and off-street parking facilities can be provided. The engineer must keep a balance between capacity and parking demand and design proper facilities.
Accident Studies
Systematic accident studies are carried out to investigate the causes of accidents and take safety measures through the design of proper facilities.
Three Stages of Accident Investigation
- Accident investigation
- Analysis of individual accidents
- Statistical analysis of accidents
Measures for Reducing Accident Rates — the Three E’s
- Engineering
- Enforcement
- Education
All three are needed together. Engineering builds a safer road, enforcement makes people obey the rules, and education makes them want to. No one of the three succeeds alone — which is exactly what the psychological factors in road user characteristics predicted.
Collision Diagrams
Accidents are recorded on collision diagrams using standard symbols for motor vehicle moving ahead, motor vehicle backing, pedestrian, fixed object, rear-end collisions, side scrape, out of control, fatal accidents, personal injury, and property damage only.
Plotting accidents at a location with standard symbols makes patterns visible immediately. A cluster of the same symbol at the same spot points straight to a specific defect — a hidden turn, a poorly placed object, an inadequate sight line.
Formula Summary
| Quantity | Expression |
|---|---|
| Volume, density and speed | Volume = density × speed |
| Capacity of a single lane | C = 1000 V / S |
| Minimum space gap | Sg = 0.278 V t |
| Minimum space headway | S = 0.278 V t + L |
| Empirical spacing | S = 0.2 V + L |
| Capacity from time headway | C = 3600 / Ht |
Quick Revision Notes
- Volume = density × speed. Maximum density means near-zero volume.
- Basic capacity = ideal conditions = theoretical capacity. Same for roads with the same physical features.
- Possible capacity = prevailing conditions; varies from zero up to basic capacity.
- Practical capacity = design capacity; the one designers use.
- C = 1000V/S and C = 3600/Ht.
- Sg = 0.278Vt; S = 0.278Vt + L; empirical S = 0.2V + L.
- 3.0 m lane gives 76 % of a 3.5 m lane’s capacity.
- Desirable lateral clearance = 1.85 m; 0.75 m one side gives 96 %, both sides 80 %.
- Six factors affecting practical capacity: lane width, lateral clearance, shoulder width, commercial vehicles, alignment, at-grade intersections.
- Capacity is quantitative; level of service is qualitative.
- Six levels A to F; speeds roughly 90, 70, 50, 40, 33, and 25–33 percent of free flow.
- LOS D is the limit of stable flow; LOS E is at capacity; LOS F is breakdown with stop-and-go waves.
- PCU converts mixed traffic to a common standard, the passenger car.
- PCU values: car 1.0, bus/truck 3.0, motorcycle 0.5, cycle rickshaw 1.5, horse drawn 4.0, small bullock cart 6.0, large bullock cart 8.0.
- Two-lane 7.0 m road = 10,000 PCU per day.
- Accident reduction measures: engineering, enforcement, education.
Mistakes Students Commonly Make
- Using basic capacity for design. Design uses practical capacity.
- Thinking maximum density means maximum volume. At maximum density, volume approaches zero.
- Forgetting the vehicle length L when computing space headway. Spacing is centre to centre.
- Putting V in m/s in Sg = 0.278Vt. The 0.278 already converts from km/hr.
- Mixing up LOS D and E. D is the limit of stable flow; E is at capacity.
- Assuming a motorcycle has a PCU of 1.0. It is 0.5.
- Assuming a bullock cart’s high PCU is about size. It is mainly about slow speed disrupting the stream.
- Treating capacity and level of service as the same thing. One is a quantity, the other a quality.
Conclusion
Capacity and level of service answer two different questions about the same road. Capacity asks how many vehicles can physically get through, and splits into three versions — the ideal, the actual, and the one worth designing for. Level of service asks what that flow feels like, and grades it from free-flowing A down to breakdown F. Add passenger car units, which make mixed Indian traffic countable in the first place, and you have the complete framework for judging whether a road is adequate for what it carries.
Frequently Asked Questions
What is traffic capacity?
The ability of a roadway to accommodate traffic volume, expressed as the maximum number of vehicles that can pass a given point in a lane or road in unit time, usually vehicles per hour per lane.
What is the difference between volume and capacity?
Volume is the actual rate of flow and responds to traffic demand. Capacity is a capability — the maximum rate of flow the road can carry at a certain level of service.
What are the three types of capacity?
Basic capacity under ideal conditions, possible capacity under prevailing conditions, and practical capacity which avoids unreasonable delay and is used for design.
Why is basic capacity called theoretical capacity?
Because it assumes the most nearly ideal roadway and traffic conditions, which means two roads with the same physical features will have the same basic capacity regardless of actual traffic.
Which capacity is used for design?
Practical capacity, which is also called design capacity.
What is the formula for theoretical maximum capacity?
C = 1000V/S, where V is speed in km/hr and S is the average centre-to-centre spacing of vehicles in metres.
What is the difference between capacity and level of service?
Capacity is a quantitative measure of how much traffic a road can carry. Level of service is a qualitative measure of operating conditions as perceived by drivers and passengers.
How many levels of service are there?
Six, designated A to F. Level A is free flow, the best condition, and Level F is forced or breakdown flow, the worst.
What average speed corresponds to Level of Service E?
About one-third of the free flow speed, with traffic volumes at or close to capacity.
What is a passenger car unit?
A common standard unit used to convert mixed traffic to an equivalent number of passenger cars, so that volume and capacity can be estimated meaningfully.
What is the PCU value of a bus or truck?
3.0, as per the IRC equivalency factors for rural roads in plain terrain.
What is the capacity of a two-lane road with a 7.0 m carriageway?
10,000 PCU per day in both directions.
What are the three measures for reducing accident rates?
Engineering, enforcement and education.