Road Safety Audit training course Motorways - safety issues of the motorway design Jesper Mertner, Road safety auditor, COWI A/S 1 ROAD SAFETY AUDITING COURSE
Content 1. Safety 2. Cross sections 3. Horizontal alignment 4. Vertical alignment 5. Safety zone 6. Barriers 7. Interchanges 8. Connection to adjacent road network 9. Rest areas 10.Pavement condition 11.Signing and marking 2 ROAD SAFETY AUDITING COURSE
Are safe Safer cross sections Conflicts seen on other kinds of roads are removed Junctions are out-of-level No vulnerable road users 3
The typical safety problems Speed variance slow moving trucks/cars and fast moving vehicles Speed important factor on number and severity of accidents Geometry (alignment) Unforgiving sides: ending of guardrail slopes in cut or fill fixed objects, etc. 4
Accidents Difficult to identify a pattern Often accidents are not concentrated But distractors, manoeuvring and weather However, typical accidents are: Loss of control left and right side often single accidents Nose to tail Some merging accidents (lane changing) When designing motorways it is important to ensure that the function of the road is respected in the design: no mixed function with vulnerable road users and farming vehicles design for the desired speed avoid too many access points 5
Design elements Cross section Alignment of road (horizontal and vertical) Split level interchanges Connection to the adjacent network Other 6
Cross section The basic cross section of a 4-lane motorway consist of: 4 driving lanes (2 each direction) Divided by a central refuge Between the central refuge and the driving lane is the edge lane. Outside the driving lanes there should be an emergency lane and a shoulder. 7
Recommendations No non-motorised traffic should be allowed. Consistency along a road is, as always, very important, so that the driver is not suddenly faced with a nasty surprise a narrow bridge for example or a sharp curve 8
Driving lanes Between 3.25 m and 3.75 m. Wider lanes will not have any or only marginally positive safety effect and narrower lanes have higher accident rates. 3.25 m only facilitate car traffic and 3.5 m facilitate trucks. Some countries allow for trucks that are 3.65 m thus a lane width of 3.75 m may be needed. 3.25 m may be used if only car traffic is using the road. Thus it may be considered to use 3.5-3.75 m in inner lane if there are trucks and 3.25-3.5 m in the outer lane. The advantage of using 2 lanes of 3.75 is the possibility to use the road with 2 lanes per direction on one side during road works by making use of emergency lane and edge lane. If this is not an option one may consider narrower lane width 9
Central refuge Guardrail should be provided on the central refuge. Even on wide central refuges there is a risk of over driving and hitting oncoming traffic On motorways it is generally recommended to use 2 one sided guardrails, however if this is not possible e.g. due to lack of space, 2-side guardrails may be used 10
Edge lane Useful to avoid the guardrail in the central refuge to be hit as there will be some time for drivers to gain control, and a person car could almost be located there. Providing edge lanes wider than 0.5 m is beneficial as studies show that increasing the width to 1.5 m reduces number of accidents by 15-20% and by increasing to 2 m by 30-35%. The marking between the outer lane and edge lane could preferable be as rumple strips to warn drivers before they unintentionally leave the road. 11
Emergency lane Important Broken down vehicles Recovery area Emergency vehicles Maintenance Rumple strips to warn drivers before they unintentionally leave the road. 12
Shoulder Typically the area outside the paved area Wider shoulders Smooth edge between the paved and unpaved The marking between the inner lane and shoulder could preferable be as rumple strips to warn drivers before they unintentionally leave the road Less safe design Safer design 13
Drainage Should be done by a crossfall on carriageway on straights of 15 25 0 / 00 A safely designed drainage system should be placed beside the carriageway with soft slopes. 14
Horizontal curve radius The higher the design speed the higher the curve radius Two following curves should have similar radius Use curves to reduce the risk of monatomic roads 15
Vertical curve radius The vertical alignment is composed of grades (slopes) and vertical curves. Steeper grades usually have high crash rates. Maximum desirable grades related to design speed are: 100km/h: 4 to 5% 80km/h: 5 to 6% 60km/h: 7 to 12% 16
Safety Zone The Safety Zone is an obstacle-free strip at the side of the carriageway. Features that are regarded as obstacles are: Embankment foreslopes (sideslopes) steeper than 1V:4H (slopes as steep as 1V:3H may be acceptable provided that there is a clear run-out area at the bottom of the embankment) Backslopes steeper than 1V:2H Non-deformable rigid obstacles, such as, bridge piers and abutments, retaining walls, rock cuttings, walls, culvert headwalls, and rigid supports for sign gantries and large signs Obstacles such as trees likely to grow to a size where the trunk width 250mm, lighting columns, sign supports having an external diameter 90mm, and equipment cabinets (structures that have been designed to be 'passively safe' (collapsible) are excluded) Ditches and open drains (unless designed to be traversable) Fences. Safety barrier! 17
Safety barriers The best solution is to remove all hazards to create a Safety Zone Safety barrier (sometimes called road restraint systems ) steel beam guardrail, wire rope, concrete walls, and gabions When the barrier works perfectly it will: prevent the vehicle from passing through the barrier (the vehicle will be contained) absorb (cushion) the impact of the vehicle without injuring the occupants (no severe deceleration) re-direct the vehicle along the road parallel to the other traffic enable the driver to retain control of the vehicle (no spinning or overturning of the vehicle) 18
Safety barriers 19
Safety barriers Safety barrier should only be used where it is likely to be safer for a vehicle to hit the barrier than if it were to hit the hazard - but this is hard to judge. Expensive to install and maintain. The best solution is to remove the hazard - installing a barrier is a second-best solution At a drop the slope can be flattened A well-established rule of thumb is to provide barrier whenever the embankment height reaches 3m The risk of a serious run-off-road crash at a particular spot increases with traffic volume, traffic speed, and road curvature, not just embankment height In the case of steel guardrail, if it is less than about 30m, the guardrail will not develop full strength 20
Guardrail terminals Out-of-control vehicles can become impaled on terminals Avoid leaving short (<80m) gaps in guardrail, so as to minimize the number of guardrail ends Fishtail ends should not be used Energy-absorbing terminal piece Flaring the guardrail so terminals are located outside the safety zone with a flare rate of at least 1 to 16 21
Interchanges For high speed roads The advantages of out of level junctions are: Eliminates most important conflict points Large capacity in main direction Easy access and limited speed reduction for main directions The disadvantages are: If not designed carefully they can be considered as complicated Difficult to place nicely - large Expensive May give detour driving May lead to new conflict if loop lead to 2 left turns instead of one right turn 22
To ensure same level of safety as on the sections it should be possible to: See the interchange in due time - guidance Understand similar design in all interchanges Have an overview of the interchange ensure visibility and sight Drive through without difficulties Good directional signage At least 3-5 km between each interchange 23
Examples of different principle types of interchanges, diamond, S, B and clover The diamond and S interchanges functions well with lower turning levels is cheaper as it requires less lanes only one bridge is needed but width should be considered carefully if channelisation may be needed later junctions with the crossing road should be considered carefully to ensure safety by e.g. using staggered junctions or roundabouts B-interchanges care should especially be taken on speed level and curve radii junctions with the crossing road should be considered carefully to ensure safety by e.g. using staggered junctions or roundabouts Clover junction mainly used when several lanes merge care should especially be taken on speed level and curve radii 24
Length of tapers and acceleration lanes Entries and exits appropriate length of tapers and acceleration lanes should be ensured 25
Connection to the adjacent network The connection of motorways to the existing adjacent should be done with care This is best done in connection with junctions and preferable roundabouts More later. 26
Rest areas Necessary to serve the long distance traffic Rest at least once every one or two hours Entrances and exits to and from Service and Rest areas can cause a disturbance for traffic on the main carriageway In interurban areas, a distance of about 50 km is appropriate The entrance to and exit from rest areas needs a special lane for slowing down and accelerating respectively. 27
Pavement condition Good pavement condition is a key safety factor Common defects are: Poor skid resistance Rutting and other deformation Pot-hole can cause loss of control Edge drops and other edge damage cause loss of control. Frequent testing of materials and mixes Regular resurfacing of shoulders are needed Quick cleaning 28
Signing and marking Rules to be obeyed at a given spot and to warn drivers of hazards Don t overload the road user with information and only chose the most important signs a warning sign or a speed limit sign is more important than a sign telling how far a given village is Never mislead the driver but give clear orientation for changes in direction and speed Visible in time correct size, clean and with reflective material Size depends on the speed and some signs have to be repeated 29
Issues to discuss: What is the most important safety issue for your motorways? What is a safety zone? How wide should the driving lanes be? What is the minimum radius on a motorway section? How should guardrail terminals be designed? 30