An operator is constructing a two-lane road section requiring a 4% crown over a total roadway width of 12 metres (excluding shoulders). If the initial passes generate a windrow of material 0.8 metres high at the centre, and the finished crown height should be achieved gradually, what is the target elevation difference from the shoulder line to the centerline for the final pass?

Question: An operator is constructing a two-lane road section requiring a 4% crown over a total roadway width of 12 metres (excluding shoulders). If the initial passes generate a windrow of material 0.8 metres high at the centre, and the finished crown height should be achieved gradually, what is the target elevation difference from the shoulder line to the centerline for the final pass?

Answer options:

• 24 cm ✅ 48 cm
• 8 cm
• 12 cm

Correct answer: 48 cm

Explanation: A 4% crown means a rise of 4 units for every 100 units of horizontal run. For a 12-metre wide road, the horizontal distance from the shoulder to the centerline is 6 metres. Therefore, the target elevation difference (rise) from the shoulder to the centerline for the completed crown will be (4/100) * 6 metres = 0.04 * 6 m = 0.24 metres, or 24 cm. The question asks for the elevation difference 'from the shoulder line to the centerline' (i.e., half the total crown rise). Oh, wait. The previous answer calculation: The total roadway width is 12 metres. A 4% crown means a 4-metre rise over 100 metres run. For 12 metres width, the total rise from one shoulder to the other, with the center being the peak (for a parabolic crown), would be (4/100) * 12 m = 0.48 m = 48 cm. The question is slightly ambiguous here. 'target elevation difference from the shoulder line to the centerline for the final pass'. Usually, a crown percentage is applied to the full width. But if it's 'from shoulder to centerline', it means half the total width. Let's re-read carefully: '4% crown over a total roadway width of 12 metres'. This typically means the rise from shoulder to shoulder is 4% of the width. So the answer for 'total crown height' would be 48 cm. If the question meant 'half crown height from shoulder to centreline', then 24cm is correct. However, standard convention applies crown percentage over the total width. Let's assume it means the highest point (centre) is 4% of 12m higher than the shoulders. So 0.04 * 12m = 0.48m = 48cm. The 0.8m windrow height is a distractor for setting the target. The 48cm refers to the total rise across the entire 12m width. Since the crown is symmetrical, this is the height of the centre above the shoulders.