15.2 Parking Supply, Duration, & Turnover Analysis

15.2 Parking Supply, Duration, & Turnover Analysis

PTOE Exam Focus: Parking analysis in Domain 5 tests a candidate's mastery of fundamental parking metrics and quantitative formulations. You must be able to calculate Parking Load ($L = \sum A_i \cdot I$), Average Parking Duration ($\bar{D} = L / V$), and Parking Turnover Rate ($T = V / S$), evaluate practical operational capacity thresholds ($85%\text{ to }90%$), and apply ITE Parking Generation methodologies for single-use and shared mixed-use developments.

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1. Core Parking Metrics & Terminology

Traffic engineers evaluate parking facilities using four foundational parameters:

+-----------------------------------------------------------------------------------+
|                         FOUR CORE PARKING PARAMETERS                              |
|                                                                                   |
|  1. Parking Supply (S): Total number of legal, marked parking stalls available.   |
|  2. Parking Accumulation (A_t): Total number of vehicles parked at time t.        |
|  3. Parking Volume (V): Total number of unique vehicles that parked over the      |
|     entire study duration: V = Initial Parked Vehicles + Sum of Inflows.          |
|  4. Parking Load (L): Total vehicle-hours of parking consumed over the study:     |
|     L = Area under the Accumulation Curve = Sum(A_i × I).                         |
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Additional Key Definitions:

• Peak Accumulation ($A_{\max}$): The maximum instantaneous number of parked vehicles observed during the survey period.
• Parking Occupancy / Utilization ($U_t$): The percentage of available parking supply occupied at time $t$ ($U_t = \frac{A_t}{S} \times 100%$).
• Parking Duration ($D$): The length of time an individual vehicle remains parked in a stall (dwell time).
• Parking Turnover Rate ($T$): The number of different vehicles that utilize each parking stall over a specified time period ($T = V / S$).

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2. Mathematical Formulations & Derivations

+-----------------------------------------------------------------------------+
|                   QUANTITATIVE PARKING EQUATIONS MATRIX                     |
|                                                                             |
|   1. Parking Load (L):                                                      |
|      L = sum_{i=1}^{k} (A_i × I)   [veh-hrs]                                |
|      Where A_i = accumulation in interval i, I = interval duration (hrs).  |
|                                                                             |
|   2. Average Parking Duration (D_bar):                                      |
|      D_bar = L / V = [sum_{i=1}^{k} (A_i × I)] / V   [hrs/vehicle]          |
|      Or from interval duration counts: D_bar = [sum (N_i × i × I)] / V      |
|                                                                             |
|   3. Parking Turnover Rate (T):                                             |
|      T = V / S   [vehicles / stall / study period]                          |
|                                                                             |
|   4. Average Space Occupancy (Utilization U_avg):                           |
|      U_avg = L / (S × T_study) × 100%                                       |
|      Where T_study = Total duration of the study period (hrs).              |
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Step-by-Step Computational Example

Consider a parking lot with $S = 100\text{ stalls}$ surveyed across a $4\text{-hour}$ study period with $30\text{-minute}$ ($I = 0.5\text{ hr}$) accumulation counts:

• Interval Accumulations ($A_1$ to $A_8$): $40, 60, 80, 90, 85, 75, 50, 40\text{ vehicles}$.
• Total unique vehicles parked over the 4 hours: $V = 260\text{ vehicles}$.

1. Parking Load ($L$): $L = \sum (A_i \times 0.5) = (40 + 60 + 80 + 90 + 85 + 75 + 50 + 40) \times 0.5 = 520 \times 0.5 = 260\text{ veh-hrs}$
2. Average Duration ($\bar{D}$): $\bar{D} = \frac{L}{V} = \frac{260\text{ veh-hrs}}{260\text{ vehicles}} = 1.0\text{ hour (60 minutes)}$
3. Parking Turnover Rate ($T$): $T = \frac{V}{S} = \frac{260\text{ vehicles}}{100\text{ stalls}} = 2.6\text{ vehicles/stall/4-hours}$
4. Average Space Occupancy ($U_{\text{avg}}$): $U_{\text{avg}} = \frac{L}{S \times T_{\text{study}}} \times 100\% = \frac{260}{100 \times 4.0} \times 100\% = 65.0\%$

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3. Practical Operational Capacity & Search Congestion

In parking planning and traffic engineering, 100% occupancy does NOT represent practical operational capacity.

• The $85%\text{ to }90%$ Threshold: When a parking facility exceeds $85%$ occupancy (and especially above $90%$), available empty stalls become scattered and difficult to find.
• Cruising for Parking: Motorists circulate through aisles searching for vacant stalls, creating queue spillback into adjacent public streets, increasing localized emissions, and creating pedestrian-vehicle conflict points.
• Design Rule: A parking facility is considered operationally saturated when peak accumulation reaches $85%\text{ to }90%$ of nominal supply. If peak demand exceeds $90%$, additional supply, shared parking management, or pricing mechanisms (demand-responsive pricing) must be implemented.

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4. Parking Study Methodologies

Transportation professionals use two primary field survey methodologies:

A. Cordon In-Out Counts (Gate Surveys)

• Observers or automated vehicle detectors (loop sensors, gate counts, optical barriers) record all inbound ($V_{\text{in}}$) and outbound ($V_{\text{out}}$) movements at all cordon entry/exit points at regular intervals ($15\text{ or }30\text{ min}$).
• Accumulation is calculated continuously from an initial baseline count ($A_0$): $A_t = A_0 + \sum V_{\text{in}} - \sum V_{\text{out}}$
• Limitation: Yields aggregate accumulation and load, but cannot determine individual vehicle dwell times or spot-specific turnover without vehicle tracking.

B. License Plate Patrol Surveys (Duration Studies)

• Observers walk or drive through the facility at fixed intervals (e.g., every $30\text{ or }60\text{ minutes}$), recording the license plate characters (or last 3 digits) and stall numbers of parked vehicles.
• Dwell time is tracked by noting how many consecutive patrol rounds each plate is observed:
  • Observed 1 time (interval $I = 30\text{ min}$): Duration $\approx 0.5\text{ hr}$.
  • Observed 2 consecutive times: Duration $\approx 1.0\text{ hr}$.
• Key Limitation: Misses "in-and-out" short-term parkers who arrive and depart within a single survey interval.

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5. Peak Parking Generation & Shared Parking Analytics

A. ITE Parking Generation Manual (5th Edition)

Similar to trip generation, parking demand is forecast using empirical rates and equations categorized by Land Use Code (LUC) and independent variable ($X$): $\text{Peak Parking Demand} = R_{\text{park}} \times X$

• Common independent variables: $1,000\text{ sq ft Gross Leasable Area (GLA)}$, dwelling units, hotel rooms, seats (auditoriums/churches), or employees.

B. Shared Parking in Mixed-Use Developments

In mixed-use developments (e.g., retail + residential + office + cinema), peak parking demands occur at different times of day and days of the week:

• Office / Employment: Peaks on weekdays between 10:00 AM and 2:00 PM; drops near zero on evenings and weekends.
• Residential: Peaks on weeknights after 7:00 PM and all day on weekends.
• Entertainment / Dining: Peaks on weekend evenings (6:00 PM to 10:00 PM).
• Shared Parking Benefit: Combining these land uses allows a shared parking facility to operate successfully with $20%\text{ to }40%$ fewer stalls than the arithmetic sum of individual peak demands.

Parking Analysis Metrics, Formulations, & Engineering Interpretations