SAFETY STANDARDS.

Auckland Star, Volume LXI, Issue 187, 9 August 1930, Page 7

SAFETY STANDARDS.

MAIN ROAD SPECIFICATIONS.

TRAFFIC REQUIREMENTS.

HIGHWAY.BOARD'S BULLETIN".

(From Our Own Correspondent.)

WELLINGTON, Friday.

Though the Main Highways Board's recent bulletin of road specifications contains matter of a purely technical nature, it makes interesting reading for motorists, since it clearly outlines the standard to which our main highways should be constructed. Practically everyone contributes to the road funds, and it is with the object of showing what we may expect to get for our money that this review of the bulletin is being made.

Increasing speed of motor vehicles, accompanied by a greater density of traffic, necessitates a more careful regard for higher construction standards Those which are laid down by the board aro designed to secure greater safety for the road user under modern conditions On all classes of roads provision is suggested in the standard specifications for generous shoulders, permitting parking of cars without unduly restricting the flow of traffic. In difficult country where a wary eye must be given to earthwork quantities, the provision of a shoulder width greater than three feet would certainly result in prohibitive construction costs, but on first-class roads the standard specification suggests that if the surfacing is 18 to 20 feet the total formation should be 24 to 30 feet. ,

Radius of Curvature. The recommended minimum radius of curvature of six chains should prove no serious hindrance to road designers except in a few extreme conditions. There is a section of the WellingtonAuckland main highway, between Shannon and Palmerston North, where the motorist has to round curves of a radius of a, chain and even less. This is admittedly a bad section, but the illustration serves to show how standards have pro-

gressed. Curves have to be super-elevated, according to a scale which has been previously explained in these columns, the factors involved in deciding the degree of super-elevation being speed and the radius of curvature. The graphs show—a warning to speedsters—that the safety margin decreases more rapidly than the speed increases.

A further refinement on curves included in the standard specifications is the provision of additional width on the inside of curves up to ten chains radius. This permits of passing at higher speeds with a safe clearance between vehicles, and also allows for the fact that the rear wheels do not follow exactly the path of the front wheels around a curve, but run inside them. Both superelevation and additional widening of curves will be introduced gradually, so that no sudden change from the normal section will be experienced when driving.

Visibility Standard. The recommended minimum visibility of 300 feet on first-class highways is somewhat generous, but it must be remembered that opposing traffic travelling at 30 miles an hour would meet at this distance in a shade over three seconds. In hilly country some difficulty might be experienced in obtaining a clear sight over the recommended distance, but this can be overcome by "benching," which means cutting back a step in the batter over which drivers may see the requisite distanee ahead. Probably the designers have forgotten the "baby" car when they recommend four feet for the height of the step.

No complaint can be found with the recommended gradients, which are well within the scope of all motor vehicles now using the roads. A question left untouched, however, is the bituminous paving of grades. It is well known that this type of paving becomes slippery in wet weather, and treacherous when coated with frost. The resulting lack of control might have serious consequences, and there was a recent instance on the Hutt Road. Wellington, when half a dozen motorists encountered trouble at the same spot, immediately they ran on to the bitumen where it was frosted.

Moderate Camber. A noticeable feature of our highway system at present is the variation of camber experienced on the same class of road, as one travels through various county sections and highway districts. The recently-published bulletin specifics within narrow limits the camber tobe given to each class of road, and it is to be hoped that these standards will be rigidly enforced. Nothing is more annoying to the motorist than to be driving on .a road where he seems in imminent danger cf sliding into the water-table. The cambers recommended are all moderate, but sufficient to provide efficient cross drainage.

Stress is laid on the introduction, at all changes of grade, of vertical curves designed to "ease the bump." It is pointed out that not only arc these a much appreciated refinement, but that they are a most, effective tool in the hands of the engineer, enabling him to follow closely the original ground level and cut earthwork down to a minimum, while still providing a first-class road.

. The bulletin contains a complete set of typical cross-sections for waterbound macadam, etc., and for each type there is a comprehensive specification.

There can be no doubt that this bulletin contains data representative of the highest standards of highway construction. Ijts general circulation will ensure unformity in our roads, though the task of bringing them up to the same standard must necessarily be accomplished gradually.