PTOE flashcards
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33 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Flow-speed-density relation
- Fundamental traffic flow equation q = k * u, where q is flow in veh/h, k is density in veh/km or veh/mi, and u is space mean speed.
- Space mean speed
- Harmonic mean of individual vehicle speeds over a roadway segment; equals total distance divided by total travel time and is always less than time mean speed.
- Time mean speed
- Arithmetic average of spot speeds measured at a fixed point; always greater than or equal to space mean speed for the same traffic stream.
- Freeway LOS density
- Level of service for basic freeway segments is determined by density in pc/km/ln; LOS A is ≤7 pc/km/ln and LOS F exceeds capacity.
- Signalized LOS control delay
- Signalized intersection LOS is based on average control delay per vehicle; LOS D is 35–55 s/veh and LOS E is 55–80 s/veh.
- Webster optimal cycle
- Optimal cycle length C = (1.5L + 5)/(1 − Y), where L is total lost time in seconds and Y is the sum of critical flow ratios.
- Saturation flow base rate
- HCM base saturation flow is 1900 pc/h/ln, adjusted for lane width, heavy vehicles, grade, parking, bus stops, and turning movements.
- Critical gap
- Minimum time headway in the major stream that a minor-stream driver will accept to complete a crossing or turning maneuver, typically 4.5–7 seconds.
- Follow-up time
- Time between consecutive minor-stream vehicles accepting the same gap; typically 2–3 seconds and used with critical gap in gap acceptance capacity models.
- Degree of saturation
- Ratio X of arrival flow to capacity for a lane group or approach; values above 1.0 indicate oversaturation and residual queues.
- Passenger car equivalent
- Number of passenger cars displaced by one heavy vehicle under prevailing conditions; increases with grade steepness and length on upgrades.
- MUTCD device requirements
- Traffic control devices must fulfill a need, command respect, convey a clear simple message, allow adequate response time, and be uniform.
- Eight-hour vehicular volume warrant
- Signal warrant based on major-road and minor-road volumes exceeding specified thresholds for each of eight hours of an average day.
- Peak hour delay warrant
- Signal warrant satisfied when minor-road traffic experiences excessive delay during the peak hour, measured by stopped delay per vehicle.
- Crash experience warrant
- Signal warrant applied when reported crashes at an intersection exceed a threshold and a signal is expected to reduce their frequency or severity.
- Roundabout entry capacity
- Entry capacity decreases as circulating flow increases; modeled via critical gap and follow-up headway in gap acceptance equations.
- 85th percentile speed
- Speed below which 85 percent of observed free-flowing vehicles travel; commonly used to set or evaluate posted speed limits.
- Pace speed
- The 10-mph speed band containing the largest number of observations in a spot speed study; a measure of speed uniformity.
- Crash rate per MEV
- Crash rate = (crashes × 1,000,000) / (AADT × 365 × years), expressing crashes per million entering vehicles.
- Poisson arrival model
- Probability of x arrivals = e^(−m) m^x / x!, where m is the mean arrival rate per interval; models random vehicle arrivals.
- Uniform delay d1
- Delay from uniform arrivals at a signal: d1 = 0.5C(1 − g/C)^2 / (1 − min(1,X) g/C), where X is degree of saturation.
- Saturation headway to flow
- Saturation flow in pc/h green equals 3600 divided by saturation headway in seconds; a 2.0 s headway yields 1800 pc/h.
- Ideal lane capacity
- Maximum flow of a single lane under ideal conditions is roughly 1900–2000 pc/h; practical capacity is lower when LOS is constrained.
- Lost time
- Time during a cycle not usable for discharge, including startup lost time and clearance lost time; subtracted from green in capacity calculations.
- Effective green
- Green plus yellow plus all-red minus lost time; the time actually available for saturation discharge in a signal cycle.
- Critical flow ratio
- Ratio of demand flow to saturation flow for the critical lane group in each phase; summed across phases to compute Y for Webster's formula.
- Progression adjustment factor
- Factor applied to uniform delay to account for platoon arrival patterns from coordinated signals; values below 1.0 reduce delay.
- Incremental delay d2
- Delay term in HCM signal delay model that accounts for random arrivals and oversaturation; grows sharply as degree of saturation approaches 1.0.
- Weaving segment capacity
- Capacity of a freeway weaving section depends on weaving ratio, segment length, and number of lanes; longer segments increase capacity.
- Density calculation
- Density k = q / u, computed from flow and space mean speed; the primary measure used to assign freeway level of service.
- Clearance interval
- Yellow plus all-red time allowing vehicles to clear the intersection; computed from intersection width, vehicle length, and approach speed.
- Saturation flow adjustment
- Product of base saturation flow and adjustment factors for lane width, heavy vehicles, grade, parking, bus stops, and turn movements.
- Spot speed study
- Field measurement of individual vehicle speeds at a point; results yield mean, standard deviation, 85th percentile, and pace speed.