Module 7 : Traffic Signal Design
Lecture 37 : Capacity and Los Analysis of a Signalized I/S
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Input parameters

To conduct operational analysis of signalized intersection, no. of input parameters are required. The data needed are detailed and varied and fall into three main categories: geometric, traffic, and signalization.
Table 1: Input data needs for each analysis of lane group
Condition Parameter
Geometric Area type
  Number of lanes, N
  Average lane width, W (m)
  Grade, G (%)
  Existence of exclusive LT or RT lanes
  Length of storage bay, LT or RT lane, L s (m)
  Parking
Traffic Demand volume by movement, V (veh/h)
  Base saturation flow rate, s o (pc/h/ln)
  Peak-hour factor, PHF
  Percent heavy vehicles, HV (%)
  Approach pedestrian flow rate, vped (p/h)
  Local buses stopping at intersection, NB (buses/h)
  Parking activity, Nm (maneuvers/h)
  Arrival type, AT
  Proportion of vehicles arriving on green, P
  Approach speed, S A (km/h)
Control Cycle length, C (s)
  Green time, G (s)
  Yellow-plus-all-red change-and-clearance interval
  (inter green), Y (s)
  Actuated or pre-timed operation
  Pedestrian push-button
  Minimum pedestrian green, Gp (s)
  Phase plan
  Analysis period, T (h)

Geometric condition

Intersection geometry is generally presented in diagrammatic form and must include all of the relevant information, including approach grades, the number and width of lanes, and parking conditions.

Traffic condition

Traffic volumes (for oversaturated conditions, demand must be used) for the intersection must be specified for each movement on each approach. In situations where the v/c is greater than about 0.9, control delay is significantly affected by the length of the analysis period.
  • If v/c exceeds 1.0 during the analysis period, the length of the analysis period should be extended to cover the period of oversaturation in the same fashion, as long as the average flow during that period is relatively constant.
  • An important traffic characteristic that must be quantified to complete an operational analysis of a signalized intersection is the quality of the progression. The parameter that describes this characteristic is the arrival type, AT, for each lane group.
    AT Description
    1 Dense platoon- 80% arrived at start of red
    2 Moderately dense- 40-80% arrived during red
    3 Less than 40% (highly dispersed platoon)
    4 Moderately dense, 40-80% arrived during green
    5 Dense to moderately dense- 80% arrive at start of green
    6 Very dense platoons progressing over a no. of closed space I/S

    The arrival type should be determined as accurately as possible because it will have a significant impact on delay estimates and LOS determination. It can be computed as

    $\displaystyle R_p = P/(g_i/C)$ (1)

    where, $ R_p$ = platoon ratio, P = proportion of all vehicles in movement arriving during green phase, C = cycle length (s) and $ g_i$ = effective green time for movement or lane group (s).

Signalization condition :

Complete information regarding signalization is needed to perform an analysis. This information includes a phase diagram illustrating the phase plan, cycle length, green times, and change-and-clearance intervals. If pedestrian timing requirements exist, the minimum green time for the phase is indicated and provided for in the signal timing. The minimum green time for a phase is estimated as,
$\displaystyle G_P$ $\displaystyle =$ $\displaystyle 3.2 + L/ S_p + 0.81 (N_{ped}/W_E)~for~W_E > 3.0 m$ (2)
$\displaystyle G_P$ $\displaystyle =$ $\displaystyle 3.2 + L/ S_p + 0.27 N_{ped}~for~W_E \leq 3.0 m$ (3)

where, $ G_p$ = minimum green time (s), L = crosswalk length (m), $ S_p$ = average speed of pedestrians (m/s), $ W_E$ = effective crosswalk width (m), 3.2 = pedestrian start-up time (s), and $ N_{ped}$ = number of pedestrians crossing during an interval (p).