PE Transportation flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Flow-speed-density equation

Traffic flow q in veh/h equals density k in veh/mi multiplied by space mean speed u in mi/h; knowing any two of the three variables determines the third in uninterrupted flow.

Peak hour factor

PHF equals the peak-hour volume divided by four times the busiest 15-minute volume; it ranges from 0.25 to 1.00 and converts hourly volumes into peak 15-minute flow rates.

Base saturation flow rate

The HCM default of 1900 passenger cars per hour per lane for a through movement at a signalized intersection, adjusted downward for narrow lanes, heavy vehicles, grades, parking, and turns.

Webster optimal cycle length

C = (1.5L + 5)/(1 − Y), where L is total lost time per cycle in seconds and Y is the sum of critical flow ratios; it minimizes average delay at an isolated signal.

Signalized intersection level of service

Determined by average control delay per vehicle: LOS A is 10 seconds or less, B is 10 to 20, C is 20 to 35, D is 35 to 55, E is 55 to 80, and F exceeds 80 seconds.

Greenshields speed-density model

A linear relationship u = u_f(1 − k/k_j), where u_f is free-flow speed and k_j is jam density; capacity occurs at half the jam density and half the free-flow speed.

Stopping sight distance

SSD = 1.47Vt + V²/[30(f ± G)], where V is design speed in mph, t is brake reaction time of 2.5 seconds, f is friction, and G is grade in decimal form.

Minimum horizontal curve radius

R = V²/[15(e + f_s)], with V in mph and superelevation e and side friction f_s as decimals; design tables set e_max and speed-dependent friction values.

Crest vertical curve length

When stopping sight distance is shorter than the curve, L = A·S²/2158, where A is the algebraic grade change in percent and S is stopping sight distance in feet.

Vertical curve K value

K equals the curve length L divided by the algebraic grade change A; it is the horizontal distance required for a 1 percent grade change and grows with design speed.

Sag vertical curve sight distance

When S is less than L, L = A·S²/(400 + 3.5S), derived from headlight sight distance with the lamp 2 feet above pavement aimed one degree upward.

Four-step travel demand model

A sequential forecast of trip generation, trip distribution, mode choice, and traffic assignment that estimates regional travel demand and produces link volumes used for planning.

Gravity model distribution

Trip interchanges are proportional to productions and attractions and inversely related to impedance: T_ij = P_i·A_j·F_ij / Σ(A_k·F_ik), where F is a friction factor.

MPO designation threshold

Federal law requires a metropolitan planning organization for each urbanized area of 50,000 or more population to conduct continuing, cooperative, and comprehensive transportation planning.

Trip generation rate

The average number of vehicle-trip ends per unit of independent variable, such as per 1,000 square feet of floor area, from ITE surveys of a specific land use.

Functional classification

Grouping roadways as arterials, collectors, or locals according to the balance of mobility and access provided; classification affects design standards, funding eligibility, and federal reporting.

Crash rate per million vehicles

R = (N × 1,000,000)/(ADT × 365 × T), where N is the number of crashes observed over T years; it normalizes crash counts by traffic exposure for site comparison.

Critical crash rate

Rc = Ra + K·√(Ra/M) + 1/(2M), where Ra is the average rate for similar sites, M is exposure in million entering vehicles, and K reflects the confidence level.

Crash modification factor

A multiplicative estimate of how a treatment changes crash frequency relative to existing conditions; values below 1.0 indicate a reduction and above 1.0 indicate an increase.

Empirical Bayes crash estimate

N_EB = w·N_predicted + (1 − w)·N_observed, with weight w = 1/(1 + k·N_predicted); it blends the safety performance function with observed counts to reduce regression-to-the-mean bias.

KABCO injury scale

A five-level crash severity classification: K fatal, A suspected serious disabling injury, B minor evident injury, C possible injury, and O property-damage-only.

Structural number

SN = a1·D1 + a2·m2·D2 + a3·m3·D3, summing each layer thickness times its layer coefficient and drainage modifier; the required SN comes from the AASHTO 1993 equation.

Equivalent single axle load

A pavement damage unit defined by an 18,000-lb single axle with dual tires; truck factors convert truck volumes into cumulative ESALs over the design period.

Resilient modulus from CBR

For fine-grained soils with a CBR of 10 or less, the resilient modulus in psi is approximately 1500 times the CBR, used to characterize subgrade support in flexible design.

PG binder grade

A performance grade such as PG 64-22 specifies the high and low pavement service temperatures in Celsius that the asphalt binder must satisfy for rutting and thermal cracking resistance.

AASHTO 1993 flexible design equation

log10W18 = ZR·S0 + 9.36log10(SN+1) − 0.20 + log10[ΔPSI/(4.2−1.5)]/[0.40 + 1094/(SN+1)^5.19], relating traffic, reliability, and serviceability loss to structural number.

Voids in mineral aggregate

VMA is the volume of air voids plus effective asphalt binder in a compacted mix as a percent of bulk volume; minimum values ensure durable, workable asphalt mixtures.

Percent within limits

A quality assurance acceptance statistic giving the fraction of tested material that falls inside specification limits, accounting for both the average and the variability of the lot.

Merging taper length

MUTCD formula L = W·S²/60, where W is the lateral offset in feet and S is the speed limit in mph, used to shift traffic around a work zone closure.

Tack coat application rate

Asphalt emulsion sprayed on an existing surface to bond pavement layers, typically applied at 0.05 to 0.15 gallons per square yard depending on surface texture and condition.

Pavement condition index

A distress-based rating from 0 to 100, where 100 is new pavement and 0 is failed; it is used to schedule maintenance and rehabilitation in a pavement management system.

Asphalt field compaction acceptance

Dense-graded asphalt mixtures are usually accepted at 92 percent or more of maximum theoretical specific gravity Gmm, measured on cores or with nuclear or electromagnetic gauges.