HCM Method of Signal Design

Lecture notes in Transportation Systems Engineering

4 August 2009

The HCM model

The HCM model for signalized intersection analysis is relatively straightforward. The model becomes complex when opposing right turns are invloved.

Input module:

The input module is simply a set of conditions that must be specified for analysis to proceed. It is the parametric description of the variables to be analyzed. Some of the variables involved are discussed here.

Area Type:

The location of the intersection must be classified as being in the central business district (CBD) or not. The calibration study conducted for the 1985 HCM $[10]$ indicated that intersections in CBDs have saturation flow rates approximately 10% lower than similar intersections in other areas. If drivers are used to driving in a big city CBD, all locations in satellite communities would be classified as ``other''. In an isolated rural community, even a small business area would be classified as a CBD. The general theory is that the busier environment of the CBD causes drivers to be more cautious and less efficient than in other areas.

Parking Conditions and Parking Activity:

If a lane group has curb parking within 84m of the stop line, the existence of a parking lane is assumed. Any vehicle entering or leaving a curb parking space constitutes a ``movement''. Where parking exists, the number of parking movements per hour occuring within 84m of the stop line is an important variable.

Conflicting Pedestrian Flow:

Left-turning vehicles turn through the adjacent pedestrian crosswalk. The flow of pedestrians impedes left-turining vehicles and influences the saturation flow rate for the lane group in question. Pedestrian flows between 1700 ped/hr and 2100 ped/hr in a cross-walk have been shown to fully block left-turners during the green phase.

Local Bus Volume:

In signalized intersection analysis, a ``local bus'' is one that stops to pick up and/or discharge passengers within the intersection at either a near or a far side bus stop. Stopped buses disrupt the flow of other vehicles and influence the saturation flow rate of the affected lane group. A bus that passes through the intersection without stopping to pick up or discharge passengers is considered to be a ``heavy vehicle''.

Arrival type:

The single most important factor influencing delay predictions is the quality of progression. The 1994 HCM model uses six ``arrival types'' to account for this impact. Arrival Type 1: Dense platoon, containing over 80% of the lane group volume, arriving at the start of the red phase. Represents very poor progression quality. Arrival Type 2: Moderately dense platoon arriving in the middle of the red phase or dispersed platoon containing 40% to 80% of the lane group volume, arriving throughout the red phase. Represents unfavourable progression on two-way arterials. Arrival Type 3: Random arrivals in which the main platoon contains less than 40% of the lane group volume. Represents operations at isolated and non-interconnected signaliazed intersections characterized by highly dispersed platoons. Arrival Type 4: Moderately dense platoon arriving at the middle of the green phase or dispersed platoon, containing 40% to 80% of the lane group volume, arriving throughout the green phase and represents favourable progression quality on a two-way arterial. Arrival Type 5: Dense to moderately dense platoon, containing over 80% of the lane group volume, arriving at the start of the green phase. Represents higly favourable progression quality. Arrival Type 6: This arrival type is reserved for exceptional progression quality on routes with near-ideal progression characteristics.

Volume adjustment module:

In the 1994 HCM module, all adjustments are applied to saturation flow rate, not to volumes. Several important determinations and calculations are done in this module.

Conversion of hourly volumes to peak rates of flow:

The 1994 HCM model focusses on operational analysis of the peak 15-minute period within the hour of interest. Since demand volumes are entered as full-hour volumes, each must be adjusted to reflect the peak 15-minute interval using a peak hour factor. This assumes that all the movements of the intersection , peak during the same 15-minute period.

Establish lane group for analysis:

Any set of lanes across which drivers may optimize their operation through unimpeded lane selection will operate in equlibrium conditions determined by those drivers. Any such set of lanes is analyzed as a single cohesive lane group. An approach is considered to be a single lane group, except for the cases of exclusive left or right-turn lanes. Where an exclusive turning lane exists, it must be analyzed as a separate lane group for analysis.

Lane utilization adjustments:

The lane adjustment made to volume is for unequal lane use. Where lane groups have more than one lane, equilibrium may not imply equal use of lanes. The 1994 HCM allows for an optimal adjustment factor to account for this. The lane utilization factor adjusts the total lane group flow rate such that when divided by the number of lanes in the group, the result is the rate of flow expected is the most heavily-used lane. When a lane utilization adjustment is used, the resulting v/c ratios and delays reflect conditons in the most heavily-used lane of the group. If the factor is not used, the resulting v/c ratios and delays reflect average conditions over the lane group.

Worksheet:

A worksheet is prepared for tabulating intersection movements,peak hour factor,peak flow rates,lane groups for analysis,lane group flow rates, number of lanes,lane utilization factor and proportion of left- and right-turns in each lane.

Saturation flow rate module:

In this module, the prevailing total saturation flow rate for each lane group is estimated taking into account eight adjustment factors. The adjustment factors each adjust the saturation flow rate to account for one prevailing condition that may differ from the defined ideal conditions.

Lane width adjustment factor:

The ideal lane width is defined as 4m, and it is for this value that the ideal saturation flow rate is defined. When narrower lanes exist, the increased side-friction between adjacent vehicles causes drivers to be more cautious, and increases headways. If width is less than 4m, a negative adjustment occurs;if width is greater than 4m, a positive adjustment occurs and if the width is equal to 4m, the factor becomes 1.00.

Grade adjustment factor:

The procedure involved assumes that the effect of grades is on the operation of heavy vehicles only, and that it is the heavy vehicles that affect other vehicles in the traffic stream. At signalized intersections, the grade adjustment deals with the impact of an approach grade on the saturation headway at which the vehicles cross the stop line.

Parking adjustment factor:

The parking adjustment factor accounts for two deleterious effects on flow in a lane group containing a curb parking lane within 84 m of the stop line: $(a)$ The existence of the parking lane creates additional side friction for vehicles in the adjacent lane, thereby affecting the saturation flow rate, and $(b)$ Vehicles entering or leaving curb parking spaces within 84m of the stop line will disrupt flow in the adjacent lane, which will further affect the saturation flow. It is generally assumed that the primary effect of a parking lane is on flow in the immediately adjacent lane. If the number of lanes in the lane group is more than one, it is assumed that the adjustment factor for other lanes is 1.00.

Local bus blockage adjustment factor:

A general adjustment factor is prescribed for the majority of ``ordinary'' bus stop situations. The model assumes that the only lane affected by local buses is the left most lane. For general cases, there is no differentiation between buses stopping in a travel lane and buses pulling into and out of a stop not in a travel lane. It is assumed that there is no effect on other lanes,i.e.the factor for other lanes is 1.00.

Area type adjustment factor:

Data collected for preparation of 1985 HCM suggest that saturation flow rates in CBDs tended to be 10% less than similar intersections in other parts of the urban and suburban area. The data were, however, not statistically conclusive and there is no algorithm for this adjustment as it depends only on the location of the signalized intersection.

Left-turn adjustment factor:

Left-turn vehicles, in general, conflict with pedestrians using the adjacent crosswalk. Left-turns may be handled under seven different scenarios.

1. Exclusive LT lane with protected LT phase (no pedestrians)
2. Exclusive LT lane with permitted LT phase
3. Exclusive LT lane with protected $+$ permitted LT phase
4. Shared LT lane with protected LT phase
5. Shared LT lane with permitted LT phase
6. Shared LT lane with protected $+$permitted LT phase
7. Single lane approach

Right-turn adjustment factor:

There are six ways in which right-turns may be handled at a signalized intersection:

1. Exclusive RT lane with protected RT phasing
2. Exclusive RT lane with permitted RT phasing
3. Exclusive RT lane with compound RT phasing
4. Shared RT lane with protected phasing
5. Shared RT lane with permitted phasing
6. Shared RT lane with compound phasing

Modelling permitted right turns:

In modelling the permitted right turns it is necessary to take into consideration,subdividing of the green phase,average time to arrival of first right turning vehicle in subject lane group,denoted by $g_{f}$,the average time for opposing standing queue clear the intersection from a multi lane approach,denoted as $g_{q}$, estimation of proportion of right turning vehicles in right lane, denoted by $P_{L}$. These parameters are to be estimated for various combinations of multilane and single-lane subject and opposing approaches.

Modelling the right-turn adjustment factor for compound (protected/permitted) phasing:

The most complicated right-turn case to be modelled is the combination of protected and permitted phasing. The factors that need to be considered are compound phasing in shared lane groups, compound phasing in exclusive right-turn lane groups, the right-turn adjustment factor for protected portion of compound right-turn phases,and the right-turn adjustment factor for the permitted portion of a compound right-turn phase. A variety of base cases can be referred to when dealing with the analysis of protected $+$ permitted or permitted $+$ protected signal phasing. In applying these procedures, manual computation becomes extremely difficult and the usage of software becomes the preferred way to implement these procedures.

Capacity analysis module:

Analysis of signalized intersection can be made through the capacity analysis module. Determining the v/s ratios, determining critical lane groups and the sum of critical lane v/s ratios, determining lane group capacities and v/c ratios, modidfying signal timing based on v/s ratios are outcomes of the procedure involved in the capacity analysis module.

Level of service module:

This involves the estimation of average individual stopped delays for each lane group. These values may be aggregated to find weighted average delays for each approach, and finally for the intersection as a whole. Once delays are determined, a level of service to each lane group can be designated and the intersection as a whole. [1]

Bibliography

1 Highway Capacity Manual. Transportation Research Board. National Research Council, Washington, D.C., 2000.