A3.9 - Determining the capacity of motorways
- A3.1 - Travel time estimation procedures
- A3.2 - The stages for estimating travel time
- A3.3 - Determining traffic volumes
- A3.4 - Calculating free speed travel time
- A3.5 - Determining the free speed of multilane roads
- A3.6 - Determining the free speed of two-lane rural roads
- A3.7 - Determining the free speed of other urban roads
- A3.8 - Determining the capacity of road sections
- A3.9 - Determining the capacity of motorways
- A3.10 - Determining the capacity of multilane roads
- A3.11- Determining the capacity of two-lane rural roads
- A3.12 - Determining whether vehicle interactions are significant
- A3.13 - Types of delay
- A3.14 - Average peak interval traffic intensity
- A3.15 - Determining the peak interval
- A3.16 - Calculating the average peak interval traffic intensity
- A3.17 - Calculating the volume to capacity ratio
- A3.18 - Calculating the additional travel time
- A3.19 - Calculating bottleneck delay
- A3.20 - Determining whether to consider peak spreading
- A3.21 - Determining the additional travel time resulting from speed change cycles
- A3.22 - Calculating the time period total average travel time
- A3.23 - Traffic signals
- A3.24 - Priority intersections
- A3.25 - Roundabouts
- A3.26 - References
A3.9 - Determining the capacity of motorways
When to use
This procedure is called from appendix A3.8.
Procedure
Following the steps below to determine the capacity of a motorway section where each direction of travel is a separate motorway section component (See appendix A2.3). Capacities are expressed as passenger car equivalents (pcu).
| Step | Action | ||
|---|---|---|---|
| 1 | Determine the basic capacity for the motorway section as follows: | ||
| If the road section has... | Then use a basic capacity of… | ||
| 2 through lanes | 4,500 pcu/h | ||
| 3 through lanes | 6,900 pcu/h | ||
| 4 through lanes | 9,600 pcu/h | ||
| 2 | Determine the passenger car equivalent to be used for trucks for the motorway section as follows: | ||
| If the terrain type is… | Then use a passenger car equivalent for trucks (Et) of… | ||
| level | 1.7 pcu | ||
| rolling | 4.0 pcu | ||
| mountainous | 8.0 pcu | ||
| 3 | Calculate the adjustment factor for trucks using the passenger car equivalent for trucks (Et) determined in step 2. Adjustment factor (ft) = 1/ (1 + Pt x (Et - 1)) where Pt = the proportion of trucks in the traffic stream during the peak period. Example: Terrain type = rolling Proportion of trucks (Pt) = 0.12 Pcu for trucks (Et) = 4.0 pcu Adjustment factor (ft) = 1/(1 + 0.12 × (4.0 - 1 )) = 0.735 |
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| 4 | Calculate the motorway section capacity by multiplying the basic capacity, determined in step 1, by the adjustment factor for trucks (ft) determined in step 3. Motorway section capacity = Basic capacity x ft Example: Through lanes = 3 lanes Basic capacity = 6,900 pcu/h Adjustment factor (ft) = 0.735 Motorway section capacity = 6,900 × 0.735 = 5072 veh/h |
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Using field measurements
If actual field measurements at the site give a different capacity from that which is determined above, then the field measurements should be used. However, if field measurements are used, then the analyst must prove that the measurements are representative of the average capacity in a variety of conditions.
Accounting for auxilary lanes
Auxiliary lanes within road sections may contribute to the road's capacity in which case the detailed procedures of the HCM shall be used. Otherwise the auxiliary lanes shall be considered not to contribute to the capacity.
