Core Concepts
Level of Service (LOS)
A qualitative measure of traffic flow conditions: A (free flow) to F (forced flow).
Traffic Volume/Capacity Ratio (v/c)
The ratio of traffic volume to road capacity. v/c < 0.7 = good. v/c > 0.9 = congested.
Trip Generation
The first step in the 4-step transportation model: estimating trips produced and attracted by zone.
Four-Step Model
Trip Generation โ Trip Distribution โ Mode Choice โ Traffic Assignment.
Worked Examples & Applications
LOS Determination: Freeway LOS C: Speed โฅ 55 mph, density 26-35 pc/mi/ln, v/c 0.70-0.80. Reasonably free flow, stable operating speeds.
Trip Generation Model: Linear regression: Trips = ฮฒโ + ฮฒโ ร Population + ฮฒโ ร Employment + ฮฒโ ร Income. Example: T = 500 + 1.2(Pop) + 0.8(Emp) + 0.5(Inc). A zone with 10,000 pop, 5,000 emp, $50k income: T = 500 + 12,000 + 4,000 + 25,000 = 36,500 trips.
Practice Questions
Q: What is the four-step transportation model? A: 1) Trip Generation โ how many trips, 2) Trip Distribution โ where they go, 3) Mode Choice โ what mode, 4) Traffic Assignment โ what route.
Q: What does LOS measure? A: Level of Service โ a qualitative measure of traffic operating conditions, from A (best) to F (worst).