PE Transportation common mistakes
PE Transportation · Understand the traps and how to avoid them.
When sizing a signal's cycle length and splits, the candidate sums the total volumes entering the intersection (or sums both approaches of each phase) instead of identifying the critical-lane volume for each phase - taking the higher volume of the two opposing through movements, divided by the number of lanes in that approach.
The words 'critical lane' and 'critical volume' appear throughout the reference material, but under exam time pressure the candidate reaches for the biggest, most familiar number on the page: the total intersection volume. Summing both approaches double counts capacity the intersection does not have.
What to do instead
Before any arithmetic, draw the phase diagram. For each phase, list the per-lane volume of each competing movement, keep only the single largest value per phase, and sum those. Only these critical-lane volumes go into the flow-ratio sum used for cycle length and green splits.
The candidate treats the peak-hour volume as the flow rate v, or applies the peak-hour factor in the wrong direction - for example dividing by PHF twice, or multiplying by PHF - and then compares that number against a capacity value expressed on a per-hour basis.
Volume and flow rate are both written in veh/h, so the distinction is invisible on the page. The candidate has memorized 'divide by PHF' without tying it to the definition of the peak 15-minute flow rate.
What to do instead
Write the definition beside the calculation every time: flow rate v = peak-hour volume / PHF. Confirm the denominator's units (a PHF must be between roughly 0.7 and 1.0), and check that the resulting v is larger than the raw peak-hour volume; if it is smaller, the factor was inverted.
On a stopping sight distance check for a grade, the candidate substitutes the grade as a whole number (4) rather than a decimal (0.04), or enters an upgrade length as positive when the equation requires the downgrade case (and vice versa) for the direction of travel being checked.
The equation is memorized as a block of symbols without the convention that G is a decimal with a sign tied to the direction of travel. Both mistakes produce an answer that looks plausible and only fails a sanity check the candidate never performs.
What to do instead
Fix the sign convention on the sketch first: for a vehicle descending, G is negative in the stopping-sight-distance equation, which increases the required distance. Convert percent to decimal before substitution, then ask whether the computed distance is longer than the level-grade value - it must be for a downgrade.
The candidate mixes unit systems in the horizontal curve relationship, using the US customary form e + f = V^2 / (15R) with a radius given in metres, or the SI form with 127 in the denominator while V is in mph.
Both equations are remembered as 'V squared over a number times R', and the constant (15 versus 127) is the only visible difference - it does not look like a units flag, so it is treated as a memorization detail.
What to do instead
Label V and R with their units before touching the equation. If any quantity is in metres or km/h, use the 127 form; if in feet and mph, use the 15 form. Also confirm whether the item asks for the maximum e or a specified e, and whether f is being taken from the design-speed table or the friction-supply equation.
The candidate uses average daily traffic directly as the design hourly volume when sizing a roadway element, computing a level of service, or estimating turning volumes, without applying the directional distribution and peak-hour factors.
ADT is the first and most prominent traffic number in the problem statement, and the design-hour variables (K and D) are often stated in a table or a following sentence. The candidate anchors on the first number given.
What to do instead
Build a small conversion ladder at the start of the problem: ADT -> design hourly volume (multiply by K) -> directional design hourly volume (multiply by D) -> peak direction lane volume (divide by lanes). Every subsequent step uses the rung the question asks about, not the bottom of the ladder.
The candidate applies a crash modification factor to the entire crash total, including crash types the countermeasure does not affect, or combines several CMFs by adding them rather than multiplying them.
CMFs look like generic adjustment percentages, and treating them like a discount applied to a total feels natural. The crash-type specificity of each CMF is a property of the countermeasure, not of the arithmetic, so it is easy to forget.
What to do instead
Before using a CMF, state in writing which crash type and site type it was developed for, and split the site's crashes into affected and unaffected groups. Multiply the affected group only. When stacking CMFs, multiply them and check whether the source permits the combination.
When comparing sites for a safety study, the candidate ranks them by crash frequency alone, or computes a crash rate using an exposure measure built from a single approach's traffic rather than all entering traffic (or from ADT rather than the period-adjusted volume).
Frequency is easy to compute and the exposure denominator feels like a formality, so it is assembled from whichever volume is nearest in the problem text. High-volume and low-volume sites then get compared on unequal footing.
What to do instead
Define the exposure quantity first - million entering vehicles for an intersection, million vehicle-miles for a segment - and write out its full construction (sum of all approach volumes x period length) before dividing. Report frequency and rate side by side; if they rank sites differently, say why.
The candidate computes design ESALs by multiplying the first-year truck traffic by the number of years in the design period, omitting the traffic growth factor, or applying the growth rate to total traffic rather than to the truck fraction.
The 18-kip equivalency arithmetic is long, and once the candidate has the first-year ESALs the design period feels like a final multiplication. The compound growth factor is a separate formula that lives in a different part of the reference material.
What to do instead
Compute three separate quantities and label them: first-year daily truck count, first-year ESALs (using the truck factor), and the growth factor [(1+g)^n - 1]/g. Multiply the last two. Sanity check: the design ESAL total for a growing roadway must exceed first-year ESALs x years.
When solving for layer thicknesses from a required structural number, the candidate divides the structural number contribution of each layer only by its layer coefficient, dropping the drainage coefficient, or assigns layers to the equation in the order they appear in the pavement section drawing rather than the order used in the design equation.
The drainage coefficient is often equal to 1.0 in simple problems, so it is treated as invisible; when it is not 1.0, the habit remains. Layer ordering is likewise copied from the sketch rather than from the equation structure.
What to do instead
Write the structural number expression out in full - SN = a1D1 + a2D2m2 + a3D3m3 - and fill in every coefficient, including m = 1.0 explicitly. Solve from the surface layer downward, carrying the remaining required structural number as you go, and verify the final thicknesses are constructible round numbers.
The candidate computes a pay quantity from plan dimensions without converting units correctly - reporting square feet where the item is paid in square yards, or converting asphalt tonnage using the wrong unit weight or a loose rather than in-place volume.
The geometry is the interesting part of the problem, so the unit conversion becomes an afterthought performed in the last ten seconds. Square feet to square yards (divide by 9) and cubic yards to tons are the two conversions most often dropped.
What to do instead
Carry units through every line of the calculation and let them dictate the conversion: ft x ft = ft^2, then divide by 9 to reach yd^2. For tonnage, write yd^3 x lb/yd^3 / 2000 with the unit weight stated as either compacted in-place or mix-design, and match that choice to the pay item definition.