Ideally capacities should be set according to those obtained from the Highway Capacity Manual or from the Highway Capacity Software or similar programs. However, separately setting capacities on every link or on every intersection approach can be quite tedious, especially considering that many of the values may change during network calibration. Many planners prefer to start with rough estimates of capacities and then to refine these estimates during calibration.
Depending upon the forecasting software, the capacities can be entered in a variety of ways. For example, UTPS and similar packages require that capacities be computed as a function of area type, facility class and number of lanes. A look-up table must be prepared giving the maximum lane volume as a function area type and facility class. The software determines the capacity of the link by multiplying the looked-up maximum lane volume by the number of lanes. Other software packages allow capacities to be set for individual links, thereby providing the user with more flexibility during calibration.
The following capacities are recommended for starting values. Where they are given as total directional capacities, they can be divided by the number of through lanes to obtain maximum lane volumes. These values should not be varied by more than ±20% unless justified by abnormal deviation from ideal conditions.
Table 12. Initial Capacities for Two-Lane, Signalized Intersection Approaches Design Capacity
(not available at this time)
Table 13. Initial Capacities for Each Lane Beyond Two, Signalized Intersection Approaches Ultimate Capacity
(not available at this time)
The initial capacities for uncontrolled road segments assume 14% trucks, 4% RV's and 0% buses, as suggested for default by the HCM for two-lane roads. The forecast period is one hour. Otherwise, ideal conditions were assumed.
Priority of signal controlled intersections relates to percent of available green time for the approach as follows: low=33%; medium=50%; high=67%. Turns relate to the percentage of traffic: low turns = 0%; high turns = 25%. The lane count does not include exclusive lanes, if applicable.
Consistency of priority should be maintained for all approaches at any given intersection. For example, it would be inappropriate to have more than two high priority approaches at an intersection.
Initial capacities for a medium amount of turns may be interpolated from the values for low and high turns.
Additional ultimate capacity for a exclusive right lane should be provided as follows for each through lane: 0 vph for low turns; 75 for medium turns; and 150 for high turns. Additional design capacity for a exclusive right lane should be provided as follows for each through lane: 0 vph for low turns; 50 for medium turns; and 100 for high turns. For example, the initial ultimate capacity for an approach with two through lanes, both exclusive left and right lanes, high priority and high turns should be 2300 (i.e.; 2000 + 2xl5O).
For signalized approaches with three or more lanes, it is necessary to extrapolate from the data for one and two lanes. For example, the initial capacity for a three lane approach with high turns, medium priority, and an exclusive left lane may be computed as follows:
|Two lanes, exclusive left, med. priority, high turns||1300|
|One lane, exclusive left, med. priority, high turns||825|
|Additional capacity for each lane beyond the first||475|
|Total capacity of three lane approach||1775|
Some-way stops are seldom included in region-wide networks. For signed approaches at a some-way stops capacity varies greatly with the amount of conflicting traffic. Ultimate capacity for each lane should not exceed 1000 vph. See Chapter 1 0 of the HCM for more information about some-way stops.
For travel forecasting packages which explicitly allow signs and signals in the network, consult the software reference manual. For example, QRS 11 requires that the capacity be set to the total saturation flow rate of the through lanes at the approach, without adjusting for signalization priority (amount of green) or amount of turning.
For links containing multiple intersections, choose the smallest capacity.
in order to obtain design capacities. The exponential term takes the fourth root of the expression in brackets; this is easily accomplished on a hand calculator by taking two successive square roots. In this equation a is between 0.56 and 1.0, depending upon the facility type (see previous discussions, Table 2 and Equation 2). This translates into values Of fold of between 0.72 and 0.62. A value of a of 0.63 (yielding a value Of fold Of 0.70) was used to construct the initial design capacities contained in the preceding sections.
The other important link attribute is the free speed. The following free speeds would be approximately correct for uncontrolled highway segments.
|Freeways and rural multilane highways|
Free speeds should not be set higher than observed speeds under uncongested conditions (LOS A).
It has frequently been observed that drivers in smaller communities choose routes as if freeways were slower than their actual speeds. Consequently, it may be necessary to reduce free speeds for freeways by a significant amount to obtain good agreement with ground counts.
The initial free speed for a long segments of uncontrolled urban streets should be set to no higher than the speed limit, unless evidence to the contrary has be obtained through spot speed studies. The initial free speeds for links containing traffic controlled intersections must be calculated from the time necessary to travel across the link and the amount of intersection delay. Perform the following steps.
Step 1. Determine the length of the link in miles, the average speed of free flowing traffic (speed limit or speed of progression, whichever is applicable), the cycle lengths of signals, and the quality of signal coordination. Express signal coordination as an "arrival type" between 1 to 5, with 5 corresponding to perfectly good progression and 1 corresponding perfectly bad progression (refer to the HCM's definitions for "arrival types"). Assume values for signalization priority according to the expected share of available green time (low=33%; medium=50%; high=67%).
Step 2. Calculate the free flow travel time in seconds. That is,tf = (3600) (link length)/(free flow speed)
Step 3. Choose a value for intersection delay in seconds, tg, from Table 17 for each signalized intersection. Use between 10 and 14 seconds for all-way stops, depending upon the amount of conflicting traffic.
Step 4. Find the total intersection delay for signalized intersections only, ts, by totaling the values of tg and multiplying by the progression factor, as indicated below.
|Arrival type 1 (poor coordination)||1.85|
|Arrival type 2||1.35|
|Arrival type 3 (no coordination)||1.00|
|Arrival type 4||0.72|
|Arrival type 5 (excellent coordination)||0.53|
Choose a value for the progression factor of 1.00, if the arrival type is unknown or if the forecast is long-term. Be sure that the signalization priority and arrival type are consistent with one another. For example, it would be unusual to have low priority for green time while also having good coordination.
Signal Timing. If signal timing is essentially unknown, then assume each signal adds 20 seconds of delay to free travel time. For different values of green time, g, and cycle length, C, the following equation from the HCM can be used to estimate delay when traffic volumes are low: (not available at this time)
Some-Way Stops. Consistency should be maintained between the capacity of a single lane at some-way stops and the delay under low volume conditions. Intersection delay is approximately,tg = 3600/(lane capacity) + acceleration delay ,
when there is little traffic approaching the sign.