Transportation Planning and Safety for the FE Civil Exam: Trip Generation, Mode Choice and Crash Rates

NCEES FE Civil spec: Transportation Engineering, E. Transportation planning (e.g., travel forecast modeling, safety, trip generation). FE Reference Handbook: Civil Engineering chapter, Transportation section (Gravity Model, Logit Models, Traffic Safety Equations).

What the FE Civil Spec Covers

Section E is transportation planning: travel forecast modeling, safety and trip generation. Problems usually ask for one step of the four-step model (trip generation rates, gravity model distribution, logit mode share), a simple traffic growth projection, AADT from counts, or a crash rate for an intersection or road segment.

Key Concepts

Trip generation: trips = rate times units (dwelling units, 1,000 ft² of floor area, employees).
Trip distribution: the gravity model splits a zone's productions among destinations in proportion to attractions and an impedance (friction) factor that falls with travel time.
Mode choice: a logit model turns utilities (negative for time and cost) into probabilities.
AADT: annual traffic divided by 365 days.
Crash rates normalize crash counts by exposure so locations with different traffic can be compared.

Core Formulas

Gravity model and logit mode choice:

T_{ij} = P_{i} \frac{A _{j} F _{ij} K _{ij}}{\sum _{j} A _{j} F _{ij} K _{ij}} P (x) = \frac{e ^{U_{x}}}{\sum _{k} e ^{U_{k}}}

Traffic growth at rate $i$ for $n$ years:

V_{n} = V_{0} (1 + i)^{n}

Crash rates:

R_{M E V} = \frac{A \times 1 0 ^{6}}{A D T \times 365} R_{M V M} = \frac{A \times 1 0 ^{6}}{A D T \times days \times length}

$R_{M E V}$ uses crashes in a single year at an intersection; $R_{M V M}$ uses crashes over the whole study period on a segment. Multiply $R_{M V M}$ by 100 for a rate per 100 million vehicle miles.

Crashes prevented with combined reduction factor $C R$ :

Prevented = N \cdot C R \cdot \frac{A D T _{a f t er}}{A D T _{b e f or e}}

Trip generation and growth

Trip generation rates (for example from the ITE Trip Generation Manual) are tied to an independent variable and a time period, such as trips per dwelling unit in the weekday PM peak hour. A 200-unit development at 0.8 trips per unit generates 160 peak hour trips. Check that the rate and the time period match what the question asks. For growth, a road carrying 1,800 veh/h today with 3% annual growth carries $1800 (1.03)^{10} = 2419$ veh/h in 10 years.

Quick distribution and mode choice examples

Zone 1 produces 1,000 trips. Zone A attracts 600 trips with friction factor 0.5 and Zone B attracts 400 trips with friction factor 1.0 (take $K = 1$ ). The weighted attractions are 300 and 400, so $T_{1 A} = 1000 (300/700) = 429$ trips and $T_{1 B} = 1000 (400/700) = 571$ trips. Zone B gets more trips despite fewer attractions because it is easier to reach.

For mode choice with $U_{a u t o} = - 1.2$ and $U_{t r an s i t} = - 1.8$ , the auto share is $e^{- 1.2} / (e^{- 1.2} + e^{- 1.8}) = 0.646$ , or about 65%. Only the difference in utilities matters, which is a useful shortcut on the exam.

On the safety side, many agencies now use the Highway Safety Manual approach, where a crash modification factor equals $1 - C R$ . A countermeasure with CR = 0.30 has CMF = 0.70: expected crashes drop to 70% of the before value.

Where It Is in the FE Reference Handbook

The Transportation section of the Civil Engineering chapter has the Gravity Model, Logit Models and Traffic Safety Equations (intersection and segment crash rates and crash reduction). Growth projections use the compound interest factors in the Engineering Economics chapter.

Worked Example: Crash Rates and Crash Reduction

(a) A 2.5 mi road segment with ADT 12,000 had 18 crashes in 3 years. (b) An intersection with 25,000 entering vehicles per day had 10 crashes last year. (c) Two countermeasures with reduction factors 0.30 and 0.20 are planned for a site expecting 20 crashes; ADT will grow from 10,000 to 11,000. Find each rate and the crashes prevented.

Step 1: segment exposure.

V M T = 12, 000 (365) (3) (2.5) = 32, 850, 000

Step 2: segment rate.

R_{M V M} = \frac{18 \times 1 0 ^{6}}{32 , 850 , 000} = 0.55 crashes per million VMT

That is about 55 crashes per 100 million vehicle miles.

Step 3: intersection rate.

R_{M E V} = \frac{10 \times 1 0 ^{6}}{25 , 000 ( 365 )} = 1.10 crashes per MEV

Step 4: combined reduction factor.

C R = 0.30 + (1 - 0.30) (0.20) = 0.44

Step 5: crashes prevented.

20 (0.44) \frac{11 , 000}{10 , 000} = 9.7 crashes

Common Mistakes

Adding crash reduction factors (0.30 + 0.20 = 0.50) instead of applying them in sequence.
Forgetting segment length or the number of years in the VMT.
Mixing rates per million and per 100 million vehicle miles.
Using simple growth (1 + ni) instead of compound growth for traffic forecasts.
Treating utilities as positive values; time and cost coefficients are negative.

Practice Problems

Try these from the free PolyCalc FE Civil problem bank. Each one has a full worked solution.

A development has 200 units and a trip-generation rate of 0.8 trips per unit for the analysis period. How many trips are generated during that period? See the worked solution or practice the Transportation deck
Annual traffic volume is 3,650,000 vehicles in a 365-day year. What is the annual average daily traffic (AADT)? See the worked solution or practice the Transportation deck
A road segment currently carries a peak-hour volume of 1,800 veh/h, and traffic is forecast to grow at 3% per year compounded. The peak-hour volume after 15 years is most nearly: See the worked solution or practice the Transportation deck
A 2.5 km road segment with an ADT of 8,000 vehicles experienced 12 crashes over three years. The crash rate per 100 million vehicle-kilometres is most nearly: See the worked solution or practice the Transportation deck

Related Calculators and Examples

Frequently Asked Questions

What are the four steps of the travel demand model?

Trip generation (how many trips each zone produces and attracts), trip distribution (where they go, often with a gravity model), mode choice (often a logit model) and traffic assignment (which routes they use).

When do I use crashes per million entering vehicles versus per million vehicle miles?

Use million entering vehicles for intersections, where exposure is the number of vehicles passing through. Use million vehicle miles for road segments, where exposure depends on segment length too.

How do I combine crash reduction factors for several countermeasures?

Do not add them. Apply each to the crashes left after the previous one: CR = CR1 + (1 - CR1) CR2 + (1 - CR1)(1 - CR2) CR3 and so on.

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