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04-For-B7 Transportation of Forest Products · May 2013

Question 4 of 6: Stopping Sight Distance

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Forest Engineering, 04-For-B7 Transportation of Forest Products, May 2013. Closed book; approved Casio/Sharp calculator only. 3 hours. Six question groups in two parts: Part A (Questions 1–4, choose 3 of 4, 60 points) and Part B (Questions 5–6, both compulsory, 60 points).

Reference texts: Heinimann, Forest Operations Engineering (forest transportation systems, vehicle/road interaction); Sessions, Forest Road Engineering Guidebook (road geometric design, stopping sight distance, road-surface traction); FPInnovations/FERIC reports (log-hauling vehicle configurations, tire-inflation systems, machine/vehicle productivity); Transportation Association of Canada (TAC), Geometric Design Guide for Canadian Roads (stopping sight distance formula, Canadian design practice); the exam's own “Required Propulsion Formulae” page (standard Davis-type railway tractive-effort and train-resistance equations, consistent with general railway/traction engineering practice, e.g. Hay, Railroad Engineering).

Check: the printed question text for Question 1 has three sub-parts (1.1, 1.2, 1.3), but the source's own mark-allocation table (page 4 of 4) lists only two 10-mark line items for Question 1 ("1.1 10 marks total", "1.2 10 marks total") -- no separate mark line for 1.3, even though every other Part A question group (2, 3, 4) prints exactly two sub-parts matching its two 10-mark line items. This looks like a table/print omission in the original exam rather than an instruction to skip 1.3 -- all three sub-parts are answered in full below, with the group shown as its 20 marks total per the printed table.

Question 4: Stopping Sight Distance (20 marks total, 4.1–4.2)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

4.1 — Finding stopping sight distance for a design speed

Stopping sight distance (SSD) is built from two additive components: the distance travelled during the driver's perception-reaction time, before the brakes are applied at all, plus the distance travelled while braking to a stop. In the form used by the Transportation Association of Canada's Geometric Design Guide for Canadian Roads (the Canadian counterpart to the AASHTO Green Book),

$$SSD = 0.278\,V\,t + \frac{V^2}{254\,(f \pm G)}$$

where $V$ is design speed (km/h), $t$ is the design perception-reaction time (a standard value of 2.5 s is used to cover the great majority of drivers), $f$ is the coefficient of friction mobilised between tire and road surface at that speed (lower for a wet or gravel/unpaved forest-road surface than for dry pavement), and $G$ is the road grade expressed as a decimal, taken as $+G$ for an uphill approach (grade helps decelerate the vehicle, shortening the required distance) and $-G$ for a downhill approach (grade works against braking, lengthening it). The first term is the reaction-distance component (constant velocity during the reaction interval); the second is the braking-distance component (derived from the vehicle's kinetic energy dissipated by tire-road friction and grade over the braking interval). The designer selects $V$ from the road's design speed, reads or assumes an appropriate $f$ for the surface and speed, applies the actual grade $G$ of the alignment, and the resulting SSD is the minimum sight distance that must be available (via horizontal and vertical alignment, and clearing sightline obstructions) at every point along the road.

4.2 — Other factors influencing stopping sight distance

Road grade -- already embedded in the SSD formula itself ($\pm G$): a downhill haul requires materially more stopping distance than the same speed on level ground, which matters directly for loaded logging trucks descending from a landing. Pavement/surface friction -- gravel, wet, icy, or loose forest-road surfaces mobilise substantially less friction than dry pavement, lengthening the braking-distance term; this is why forest-road SSD design typically assumes a lower $f$ than a paved-highway standard would. Vehicle mass and brake type -- a fully loaded logging truck has far more kinetic energy to dissipate than an empty vehicle at the same speed, and heavy-vehicle air-brake systems have their own additional brake-lag time beyond the driver's perception-reaction time, both of which lengthen the effective stopping distance beyond the passenger-vehicle design value if not separately accounted for. Driver perception-reaction time -- affected by fatigue, alertness and experience, which is a real operational concern on long logging-truck shifts. Sight-distance obstructions -- horizontal curvature (cut banks, roadside vegetation or timber on the inside of a curve), vertical curvature (crest curves hiding the road ahead), and the assumed driver eye height / object height used in the design standard, all of which determine how much of the theoretically required SSD is actually available to be seen, independent of the braking physics.