Visibility (traffic planning)

A lack of visibility has a negative effect on road safety.

Sufficient visibility , which enables the driver of a vehicle to grasp the route and the route characteristics in the direction of travel and to regulate his driving behavior accordingly, is a requirement that must be met over the entire route in both directions in order to ensure traffic safety . Therefore, this must be taken into account when designing roads. The visibility on the open road should not be confused with the visibility at the junction . There the approach view , the approach view and the stop view are used as dimensioning criteria.

The determination of the visibility on free routes and the dimensioning criteria are supported by the guidelines for the construction of roads; - Part of the lines (RAS-L), 1995 edition of the Research Association for Roads and Transport . In the meantime, however, the standard has been revised, which means that parts of this article are no longer up-to-date. Current information on visibility in traffic planning can be found in the respective guidelines, depending on the traffic route category: Guidelines for the construction of country roads (RAL), guidelines for the construction of city streets (RASt 06) and guidelines for the construction of motorways (RAA).

Stopping visual range

The stopping visual range is the distance a driver needs to bring his vehicle to a stop in the event of unexpected obstacles on the road.

Relevant speed

The speed v ₈₅ , which is derived from the design speed v _{E, is} used as the base speed for assessing visibility . The v ₈₅ is the speed that describes the actual driving behavior, which is not exceeded by 85% of the unhindered driving cars on wet roads. The following formula is used for single -lane cross-sections of road category A ( roads without cultivation):

v ₈₅ = v _E + 20 km / h with v _E <100 km / h and v _{85, max} = 100 km / h

Actual visibility

Before evaluating the visibility according to the criteria of stopping visibility and overtaking visibility, the actual visibility must be determined in sections and separately for both directions of travel. Similar to the curvature of tape and at the bank band a is to view wideband drawn, the horizontal X-axis from the stationing (added lengths) of the axle elements is determined. The visibility in both directions of travel is plotted perpendicular to this on a suitable scale and connected by lines with different dashed patterns. Since the visibility also depends on the course of the gradient of the axis , the visibility band is drawn on the elevation plan in the same length scale and can then be directly compared with the gradient and curvature display.

The stopping visual range is determined in two stages:

• In the map , a line is drawn which is 1.80 m in parallel to the left edge of the lane. A so-called “eye point” or “eye point” is selected on this line, ie a point that roughly corresponds to the position of the driver. A straight line is constructed from the point of view , which at the critical point is just within the width of the carriageway plus verges if there are no restrictions in the field of vision due to guard rails or structures. The distance from the eye point to the intersection of this straight line with the selected auxiliary line is the visual range in the site plan. The lane in the direction of travel is decisive for each direction of travel, although the line of sight can be in the area of ​​the entire lane. The sequence of the eye points - depending on the curviness of the route - is chosen so dense that a linear development of the visibility from eye point to eye point is approximately given.
• The visibility from the site plan, i.e. the horizontal visibility, must be checked for vertical restrictions using the gradient, mainly in the area of hilltops . The following conditions apply here:

1. Height of eye points above the road: 1.00 m (for cars)
2. Height of target points above the road: 0.00 m (v ₈₅ = 60 km / h) to 0.35 m (v ₈₅ = 100 km / h)
3. The connecting line eye point - target point must not fall below the height of the gradient anywhere in this section.
   If necessary, the raw visibility from the site plan must be corrected to this value. In the area of tubs , it may also be necessary to check the clearance height of crossing bridge structures with this method.

Checking the stopping distances

The calculation of the necessary stopping visual distances must also be carried out separately for both directions of travel and in sections, as they depend on the longitudinal inclination . The RAS-L contains a diagram for taking the values ​​and gives the following approximate formulas for a rough calculation:

${\displaystyle s_{h}=s_{1}+s_{2}\quad }$		required stopping distance [m]
${\displaystyle s_{1}=v_{85}\cdot {\frac {t_{R}}{3{,}6}}\quad }$		Path of reaction and impact duration [m]
${\displaystyle s_{2}={\frac {1}{2g\cdot 3{,}6^{2}}}\cdot {\frac {v_{85}^{2}}{f_{T}+{\frac {s}{100}}}}\quad }$		pure braking distance [m]
${\displaystyle f_{T}=0{,}241\cdot \left({\frac {v_{85}}{100}}\right)^{2}-0{,}721\cdot {\frac {v_{85}}{100}}+0{,}708\quad }$		tangential adhesion coefficient [-]

${\displaystyle v_{85}}$		[km / h]	Speed ​​v 85
${\displaystyle t_{R}}$		[s]	Response and impact time (t R = 2 s)
${\displaystyle g}$		[m / s 2 ]	Acceleration due to gravity (g = 9.81 m / s 2 )
${\displaystyle {\tfrac {s}{100}}}$		[-]	pitch

The values ​​of the necessary stopping distances are plotted analogously to the actual stopping distances. In no area of ​​the route may the actual stopping visual distances fall below the line of the necessary stopping visual distances.

Effect on the route design

Even in the early design phase, the axis and gradient design must also be checked with regard to the visibility during fine-tuning. If there are conflicts here, the axis and gradient must be changed in such a way that they are eliminated. Often it is sufficient to only keep an eye on a few sections of the axis, because the problem areas with regard to the visibility are relatively easy to see. In the map above all the route sections whose design parameters close to the lower limit of the chosen design speed v are _E lie. In the profile plan, this applies to the rounding of the hilltops. The fine-tuning and optimization of the axis of a traffic route is often a lengthy, iterative process, especially when there is a high "pressure to adapt" due to difficult terrain to design a route with good driving dynamics, economical execution and maintenance and good integration into the landscape.

Overtaking visibility

In addition to the stopping visual range, the overtaking visual range is important in the design of the spatial routing of traffic routes, but in this case as an option that enables a good flow of traffic. The required range of vision includes a fully visible route over a length that enables the entire overtaking process with sufficient safety distance.

The determination of the overtaking visibility and its assessment criteria are specified in the guidelines for the construction of roads; - Part of the lines (RAS-L), 1995 edition of the Research Association for Roads and Transport.

Relevant speed

As in the case of the stopping visual range, the speed v ₈₅ , which is derived from the design speed v _E , is used as the base speed for assessing visibility .

Checking the overtaking visibility

The RAS-L determines the required overtaking visual range depending on the speed v ₈₅ , as the table on the right shows.

Overtaking maneuvers and overtaking visibility

Overtaking visibility as a function of speed v 85 v 85 [km / h] Overtaking visibility [m] 60 475 80 525 100 625

These visibility ranges are based on the following model:

• The vehicle to be overtaken travels at the speed . ${\displaystyle 0{,}85\cdot v_{85}}$
• The oncoming vehicle travels at speed v ₈₅ .
• Passing vehicle accelerates from v ₈₅ on and shears on the counter lane from. There the vehicle maintains this speed until the slower vehicle is overtaken. It then cuts into its lane again and has the required safety distance to both the oncoming and the overtaken vehicle. ${\displaystyle 1{,}10\cdot v_{85}}$

Sufficient overtaking visibility is available if the actual visibility is equal to or greater than what is defined as the minimum length in accordance with v _{85 is the} specified overtaking visual range. The design of the overtaking visibility range is determined in the same way as the stopping visibility range; the only difference is that the overtaking visibility range is derived from the field of vision to the opposite direction of travel.

Consideration of the overtaking visibility when designing the route

In contrast to the stopping visual range, which must be available over the entire length of the route, a route portion of 20 to 25% of the total route is sufficient for safe overtaking, whereby the distribution of these areas should be approximately even. If this section of the route cannot be increased by changes in the axis and gradient design, because this is e.g. B. is not possible for reasons of economy and landscape protection , an additional lane may help. Such sections can be fully counted towards the overtaking areas in the respective direction of travel, since overtaking maneuvers do not affect oncoming traffic here .

When planning the design, the following points must be observed with regard to the overtaking visibility:

• The overtaking visibility range is to be checked especially on roads of category A that are not cultivated and used in two-way traffic, and must be included in the design criteria. In the case of paved roads of categories BI and B II, there is no entitlement to overtaking visibility for reasons of traffic safety (turning traffic).
• Visibility ranges in the range of half to full overtaking visibility lead to incorrect assessments that are detrimental to road safety.
• If overtaking is prohibited in such areas, it must be checked whether the slow agricultural traffic can be overtaken safely. If this is the case, the overtaking of these vehicles can be approved by means of additional signs.
• If the visibility is limited, the spatial alignment must be checked very carefully. If parts of the route "dive" below the vertical visual range within the scope of the actual visual range and then emerge again without a clearly recognizable offset in the area of ​​the overtaking visual range, dangerous overtaking maneuvers due to incorrect assessment of the situation are possible. Here, the lines must be changed so that this is no longer the case.
• The overtaking visibility range is not a primary design criterion, but for reasons of traffic safety it should be included in planning in such a way that the recording of the route from the driver's point of view leaves no room for misinterpretation. Overtaking bans may be necessary, but should not be foreseen in advance. They lose a lot of effect if their purpose is not also recognizable from the route.

Norms and standards

• Research Society for Roads and Transport: Guidelines for the construction of country roads - RAL 2012 , edition 2012, FGSV-Verlag Cologne
• Research Society for Roads and Transportation: Orientation Visibility - Definition and Assessment , 2007 edition, FGSV-Verlag Cologne

Individual evidence

1. ^ Henning Natzschka: Road construction - design and construction technology, BG Teubner Verlag, 2003, ISBN 3-519-15256-8 , page 153
2. ^ Günter Wolf: Strassenplanung , Werner Verlag, 2005, ISBN 3-8041-5003-9 , page 199
3. ↑ Henning Natzschka: Road construction - design and construction technology, BG Teubner Verlag, 2003, ISBN 3-519-15256-8 , page 154
4. ^ Günter Wolf: Strassenplanung , Werner Verlag, 2005, ISBN 3-8041-5003-9 , page 203