The minimum headroom above the carriageways is at least equal to the maximum (design) height of heavy good vehicles (HGV) that are allowed on the road, with additional clearance necessary to allow for movements of the vehicles due to irregularities of the pavement and the vehicle.
The minimum headroom depends on the maximum height of heavy good vehicles and varies from country to country. In most European countries the maximum height of heavy good vehicles is 4.0 m; certain countries allow higher values (UK, USA): see table 7.1 in Chapter 7 "Maintained headroom" of the Report 05.11.B.
In the European Union the maximum height of heavy good vehicles is 4.00 m, although the Geneva conventions allow a maximum of 4.3 m. If a margin of 0.20 m is added to these maximum heights in order to absorb vertical movements of the HGV, the minimum vertical clearances required are 4.20 m (4.50 m).
Above these minimum clearances, additional headroom is necessary for drivers of HGV's to feel comfortable. This comfort margin is related to the object distance. The minimum height plus the comfort margin yields the maintained headroom. If a value of 0.30m is taken for the comfort margin, the maintained headroom is 4.50 m (Geneva convention 4.80 m, UK 5.35 m, USA 4.90 m on freeways, 4.30 m on other highways).
To prevent damage of equipment mounted above the carriageway by loose tarpaulins for instance, an additional allowance is often applied.
Finally, allowance has to be made for inaccuracies in the construction, bending of the roof and possible later paving overlays see Chapter 7 "Maintained headroom" of the Report 05.11.B and Chapter 7.8 "Vertical clearances" of the Report 05.12.B.
The specific case of the geometric design of reduced height urban tunnels is treated separately, as they are normally reserved to cars and some restricted categories of (light) vans.
The full study has been made for France and implies the following specific points due to the presence mainly of cars, available in the article "Reduced height urban tunnels geometric design" (Routes/Roads 288 - 1995):