Junction Collision: Reconstructing Speed from Skid Marks
At 21:40 on a dry asphalt approach to a signalised T-junction in Vijayawada, a 150 cc motorcycle strikes a pedestrian who is inside the marked crosswalk. The motorcycle leaves a continuous front-tyre skid mark 18 m long before the point of impact and a further 4 m of scrape after impact. The rider, who holds a learner's licence issued eleven days earlier and is riding alone, tells the inspector he was doing 'about 35 km/h'. The posted limit is 50 km/h. The road is level, the tyres are inflated to specification, and the investigating officer records a peak braking coefficient of 0.65 on this surface. A CCTV camera covers the junction but its time stamp is 3 minutes fast. The pedestrian's signal phase is disputed. The rider has no injury; the pedestrian has a fractured tibia. The motorcycle's front brake lever is bent but the brake still operates.
1. Reconstruct the minimum speed at the start of the skid and test the rider's 35 km/h claim. State every assumption that affects the result.
Read the worked answer
Use the skid-to-stop relation v = sqrt(2 * mu * g * s). With mu = 0.65, g = 9.81 m/s² and s = 18 m, v = sqrt(2 × 0.65 × 9.81 × 18) = sqrt(229.6) = 15.15 m/s = 54.5 km/h. This is the minimum speed at the instant the front tyre began leaving the continuous mark; the actual speed could be higher if not all tyres were braking or if the mark began after some deceleration. The 4 m after impact is scrape/slide evidence and must not be added to the braking distance. At 35 km/h (9.72 m/s), the same coefficient would produce only v²/(2 mu g) = 9.72²/(2 × 0.65 × 9.81) = 7.4 m of skid, less than half the observed 18 m. The 35 km/h claim is therefore inconsistent with the physical evidence. If a 1.0 s perception-reaction time is assumed, the motorcycle was about 15.15 m upstream when the hazard became perceptible and the total perception-to-stop distance was about 33.15 m. State that the result assumes level dry asphalt, both brakes effective, no ABS cycling, and a constant coefficient; a brake inspection and a test on the actual surface should confirm mu.
2. What offences and licence issues arise, and which are supportable on the present evidence?
Read the worked answer
The speed reconstruction supports a prima facie case of exceeding the 50 km/h limit, because the minimum pre-braking speed of 54.5 km/h is already above the limit. Section 183 of the Motor Vehicles Act, 1988 covers excessive speed. Section 184 (dangerous driving) may also be considered if the evidence shows the rider failed to slow for a marked crosswalk and a pedestrian already on it; speed alone does not automatically prove dangerous driving, so the pedestrian's position, signal phase and visibility must be proved. The rider's learner's licence is not an effective licence for solo riding: a learner must be accompanied and display the prescribed 'L' board, so proceeding under Section 181 for driving without an effective licence is supportable if the licence conditions were breached. Do not charge both Section 181 and Section 3 in a confused way; record the licence category, issue date, endorsement and the absence of a companion. Insurance and registration should be checked separately. The bent brake lever should be photographed and the brake tested, but it does not by itself prove a brake defect.
3. How should the disputed CCTV timestamp and pedestrian signal phase be handled so the evidence survives scrutiny?
Read the worked answer
Do not rely on the visible 3-minute error as if it were a fact. Seize the original DVR or its native file with hash value, record the chain of custody, and note the recorder's clock offset against a reliable reference such as the police control-room time or a mobile network time display captured on video. Obtain the signal controller's phase log for the junction, because the controller log is more reliable than the video overlay. Correlate the pedestrian's position, the motorcycle's entry into the frame, and the signal phase frame by frame. Interview independent witnesses and record whether the pedestrian had started crossing on a pedestrian green or had overstayed. If the signal phase cannot be proved, the case should rest on the speed and failure-to-slow evidence rather than on an unproved signal violation. The 3-minute offset should be documented as a known offset, not ignored, and any edited clip should be excluded in favour of the original.