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Information Disclaimer: This is an unofficial collection of European road sign conventions and rules. Whereas every effort has been taken to ensure the accuracy of the information presented, there may be errors and omissions.The entire risk as to results and performance of the information rests with its user.

Approaching Certain Curves

Arithmetic for Curves in Partially Obstructed View

There are many scenic stretches of highway or Autobahn (autoroute, autostrada, autopista, motorway). Some may lead through forests, others curving up or down hills. Well, suddenly, near the end of a curve impossible to oversee, we come upon a line of stopped cars. Slam on the brakes, hoping for the best (?...)

Beware also of the fact that, generally, European traffic lanes are narrower than those in America. So is decidedly the right of way (i.e. strip of land) cleared of obstructions. Also, there may be no shoulder. From geometry and arithmetic follow practical consequences, some of which we discuss below.

Traffic density, traffic patterns, accidents, and other elements of probability cause traffic to get stopped off sometimes. So, while we may be travelling along happily, it behooves us to consider just how far ahead a stopped car, or line of cars, can be seen in a right curve, assuming this to be in a wooded stretch of horizontal highway.

Lane width of major highways and expressways is typically 3.5 m. Let us assume there is a clearance of 5.0 m between the highway and the forest boundary that obstructs our view, limiting it to the entry portion of the curve ahead.

In the case of a typical highway, we can assume one lane in each direction, and the stopped car - or line of cars - to occupy the right half of our lane. In the case of an Autobahn (autoroute, autostrada, autopista, motorway) we assume two lanes going in our direction and the stopped cars to occupy the right half of each. What, then, is the maximum safe speed at which we can stop within the free path to the stopped vehicles ahead? (that is within the path visible geometrically, in the Table below). Because, from the geometry of road and forest boundary, the overseeable distance is shorter in a right turn than in a left turn, we use the right turn for this discussion. Of course, sides change in GB and IRL.

Our results, tabulated below, are based on our reference deceleration of -2.5 m/s2 , and the reference of 1 second for reaction time and brake response. Of course the pavement must be reasonably smooth, clean, and dry to withstand -2.5 m/s2 . For a practical definition of it, see "Stopping distances" in the"Table of Contents" of the previous page. Return to it.

Table: Radius of road curvature versus visibility ahead and safe speed


Radius of curvature (m)

Geometrical visibility ahead (m)

Safe speed (km/h)

highway 300 128 83
highway 400 147 89
highway 500 165 95
Autobahn, motorway 500 182 100
Autobahn, motorway 750 224 117
Autobahn, motorway 1 000 258 121

Conclusion: Beware!

Of course, traffic engineers are alert to our facts and reasonable speed limit signs are usually posted, often with Zusatzschild (info plate), such as "bei Nässe" (when wet). A similar purpose is served by the traffic-congestion warning-sign. See "Warning Signs" in the"Table of Contents" of the previous page (check below).

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Last revision of this page: 07.11.97: H.F.V.