Transportation basics

Why it matters

Transportation engineers design roads, intersections, transit, bike and walking facilities so people move safely and efficiently. Two ideas sit underneath much of that work: drivers need enough sight distance to see a hazard and stop, and facilities are sized from traffic counts. Even site-development engineers use both, for driveways, parking lots and turn lanes.

Design references

In the US, geometric design follows AASHTO's A Policy on Geometric Design of Highways and Streets (the "Green Book", 7th edition 2018, with newer editions in progress), plus state DOT manuals. Traffic control devices (signs, signals, markings) follow the FHWA Manual on Uniform Traffic Control Devices (MUTCD), 11th edition (https://mutcd.fhwa.dot.gov). Capacity analysis uses the Highway Capacity Manual (HCM) from the Transportation Research Board. Checked on 2026-10-23. Use the editions your agency requires.

Stopping sight distance

Stopping sight distance (SSD) is the distance a driver needs to perceive a hazard, react, and brake to a stop. It has two parts:

SSD = reaction distance + braking distance
SI: SSD (m) = 0.278 * V * t + 0.039 * V^2 / a

V is the design speed in km/h, t the perception-reaction time, and a the deceleration rate. The Green Book's design values use t = 2.5 s and a = 3.4 m/s^2, chosen to cover most drivers on wet pavement. On grades, braking distance changes: downhill is longer, uphill shorter.

Worked example. At 100 km/h:

• Reaction: 0.278 × 100 × 2.5 = 69.5 m
• Braking: 0.039 × 100^2 / 3.4 = 114.7 m
• SSD = 184 m (design tables round this to about 185 m).

Doubling the speed roughly quadruples the braking part, which is why speed management is central to road safety.

Where SSD shows up

• Crest vertical curves must be long enough that a driver's eye (about 1.08 m high) can see an object on the road (about 0.60 m high) at SSD.
• Sag curves at night are limited by headlight reach.
• Horizontal curves need the inside of the curve kept clear of buildings, cut slopes and vegetation.
• Intersections and driveways need sight triangles so drivers can see approaching traffic (intersection sight distance, a related but separate check).

Other sight distances include decision sight distance (complex locations) and passing sight distance on two-lane roads.

Traffic counts

Traffic volumes are counted with tubes, loops, video or radar, and summarised as:

• AADT (annual average daily traffic): total vehicles per year divided by 365. Short counts are adjusted to AADT with seasonal and day-of-week factors.
• K factor: the fraction of AADT in the design hour (commonly somewhere around 8 to 12% on many roads; use local data).
• D factor: the fraction of design-hour traffic in the peak direction.
• Design hour volume in the peak direction: DDHV = AADT × K × D.
• Peak hour factor (PHF): how uneven flow is within the peak hour, PHF = hourly volume / (4 × peak 15-minute volume). A PHF of 1.0 means perfectly even flow; lower values mean a sharper peak.

Worked example. AADT = 20,000 vehicles/day, K = 0.10, D = 0.55: DDHV = 20,000 × 0.10 × 0.55 = 1,100 vehicles/hour in the peak direction. If the peak hour carried 1,200 vehicles with a busiest 15 minutes of 360, PHF = 1,200 / (4 × 360) = 0.83.

These volumes feed the HCM's level of service (LOS) analysis, graded A (free flow) to F (breakdown), which drives lane counts, signal timing and turn-lane decisions. Vehicle mix matters too: trucks use more capacity and need longer stopping and turning space.

Beyond cars

Modern practice designs for everyone: complete streets, pedestrian crossings and accessible routes (ADA and the PROWAG accessibility guidelines), bike facilities, transit stops, and safety programs such as the Safe System approach adopted by FHWA. Crash data analysis (the Highway Safety Manual) identifies locations and treatments.

Common mistakes

• Using the posted speed instead of the design speed.
• Forgetting grade in braking distance.
• Treating a single day's count as AADT without seasonal adjustment.
• Mixing directional and two-way volumes in capacity analysis.

Professional responsibility

Road and traffic designs are reviewed by agencies and sealed by licensed engineers, often with a professional traffic operations engineer (PTOE) involved. Real designs need review and sealing by a licensed professional engineer (PE) under the codes your jurisdiction adopts. Collecting counts on live roads needs traffic-safety training and approved traffic control.

Recap

• SSD = reaction distance + braking distance; at 100 km/h it is about 185 m with Green Book values.
• Sight distance shapes vertical and horizontal curves and intersection layouts.
• AADT, K, D and PHF turn counts into design hour volumes for capacity and LOS analysis.