Photos of cars on freeway, speeding sign

Freeway Management and Operations Handbook

Chapter 5 – Roadway Improvements

5.1 Introduction

An oft-repeated axiom of this Handbook is that freeway practitioners must view the overall performance of the transportation network as a whole; broadening their view of "management and operations" to include other approaches for improving freeway performance that have not traditionally been considered their responsibility. This expanded view means looking beyond the "typical" freeway management and operation alternatives (e.g., ramp management and control, managed lanes and HOVs, traffic incident and planned special event management, traveler information dissemination, traffic management centers, surveillance, etc; as discussed in subsequent chapters), and giving consideration to other types of improvements in concert with freeway management systems and strategies. These additional strategies and improvements may include increasing capacity at bottleneck locations, altering the geometrics to eliminate safety hazards, enhancing various attributes of the freeway environment (e.g., signing, pavement markings, illumination) to increase safety and driver convenience, and implementing strategies to reduce travel demand.

The introductory chapter to this Handbook uses the analogy of a three-legged stool to describe effective highway transportation. This stool consists of three component parts – building the necessary infrastructure, effectively preserving that infrastructure, and effectively preserving its operating capacity by managing operations on a day-to-day basis – with all three parts, or legs, existing in the appropriate proportion to one another. Thus, the freeway practitioner needs to somewhat "blur" any distinction between these "legs", considering improvements to the infrastructure to be within the broad realm of "operations".

5.1.1 Purpose of Chapter

This chapter provides a high-level overview of potential actions that improve freeway performance by modifying the roadway itself, such as adding lanes to increase capacity (and thereby increase operational efficiency) at roadway bottlenecks, ramps, interchanges, or other roadway locations; and making changes to the geometric configuration or physical characteristics of the roadway to enhance safety. After a brief overview of the types of problems that can be addressed by roadway improvements (and the potential benefits), and how these potential improvements should be addressed within the freeway management program, the following improvements are discussed: horizontal and vertical alignment; roadway widening (e.g., auxiliary lanes, shoulders); providing additional lanes without widening (e.g., restriping, use of shoulder as travel lane); interchanges (improvements to ramps and weaving sections); and other improvements such as treatment of obstacles and skid resistance.

It is emphasized that this chapter provides only an introduction to possible roadway improvements in support of freeway operations. For additional details and design guidelines, the practitioner should consult a variety of references, many of which are identified at the end of this chapter. Moreover, new freeway facility construction, major 3R projects (resurfacing, restoration, rehabilitation), and significant freeway infrastructure construction (e.g., new interchange, widening over a stretch of several miles) are not addressed herein; although the practitioner should nonetheless be cognizant of the potential of such major improvements, and consult the appropriate references as required.

5.2 Background and Overview

Chapter 1 discusses the concept of "recurrent congestion" – a situation that occurs when demand increases beyond the available capacity of the roadway. It is usually associated with the morning and afternoon work commutes, when demand reaches such a level that the freeway is overwhelmed and traffic flow deteriorates to unstable stop-and-go conditions. The obvious solution to this problem – and often the most effective – is to increase the capacity of the affected segment.

Several physical attributes of the freeway facility impact its capacity and operational characteristics as summarized in Table 5-1. Additional factors effecting capacity include percentage of heavy trucks, level of speed enforcement, lighting conditions, pavement conditions, pavement markings and signing, and weather. By enhancing one or more of these roadway elements, the capacity will be increased thereby improving traffic flow. As an example, using the methodologies and tables contained in the Highway Capacity Manual (Reference 1), increasing the number of freeway lanes from 2 to 3 will increase the "service volumes" (Note: For urban conditions, 12-foot lanes, 6-foot shoulders, level terrain, 5% heavy vehicles, and 1 interchange per mile) at Level of Service D from 3840 vph to 5850 vph – an increase of a little over 50%. Another example – cited in Reference 2 – involves increasing the distance to obstructions on both sides of the freeway (having two 12-foot lanes in each direction) from one foot to six feet could increase the capacity by about 10%.

Table 5-1: Physical Factors Affecting Roadway Capacity and Operations
(Reference 1)
Category Capacity / Design Element
Horizontal Alignment
  • Degree of curvature
  • Super elevation
Vertical Alignment
  • Grade
  • Length of grade
  • Vertical curves – sag and crest
Cross Section
  • Number of lanes
  • Lane width
  • Lateral Clearance
    • Shoulder type and width
    • Median type and width
    • Clearance to obstructions
  • Interchange density
  • Ramps & ramp junctions
  • Weaving sections

Roadway improvements can also enhance traveler safety by improving hazardous locations. In fact, the ITE Document "A Toolbox for Alleviating Traffic Congestion and Enhancing Mobility" (Reference 2) includes a table listing the "design elements" that can influence safety. It is essentially identical to Table 5-1 above (with the addition of sideslopes, roadside traffic barriers, and ditch design). A few examples of roadway improvements enhancing traveler safety include:

  • In certain cases, an improperly aligned highway with sharp curves may have five times as many accidents compared to a highway with good alignment (Reference 2).
  • As reported in Reference 3, Caltrans has evaluated many of its safety projects to determine what has been effective. On average, curve correction was found to reduce 50% of all accidents, superelevation correction reduced 50% of run-off-road accidents, and truck escape ramps reduced 75% of run-away truck accidents.
  • Longitudinal grooving of pavements has shown dramatic reductions in wet pavement accidents. The FHWA Report "Effectiveness of Alternative Skid Reduction Measures" (Reference 4) references two California studies from the early 1970's where grooving resulted in reductions in the wet pavement accident rates of 70 and 73%, with the largest decreases in sideswipe, fixed object, and rear-end accidents. The same reference summarizes 77 grooving projects in 13 states that showed an overall decrease of 75% in wet pavement accidents.

5.2.1 Key Considerations During Freeway Management Program Development

It is important for freeway practitioners to regularly address and evaluate a full array of freeway improvements – from large-scale projects to "low cost" roadway enhancements (Note: Low cost relative to building a new roadway or widening long stretches of existing facilities) as potential elements of a freeway management and operations program. As such, they should be considered throughout the various activities that comprise the development and management of a freeway operations program (refer to Chapter 3).

A critical issue (or "step" as shown in previous Figure 3-1) is that of the "institutional environment". One of the major differences with many of the actions discussed in this chapter compared to the operational improvements discussed in subsequent chapters is that, in almost all cases, roadway improvements that add capacity are subject to planning and environmental requirements (Note: Some freeway practitioners have indicated that, in their experience, the installation of auxiliary lanes for freeway entrances and exits, and the widening of entrance ramps have not been subject to these requirements; or when they are, the result is a "Negative Declaration", allowing the roadway improvement to move forward without going through a time consuming environmental study.) that must be followed to secure financial support (2). This may include:

  • A Major Investment Study (MIS), where a major transportation investment is identified through the planning process as satisfying a need and where federal funds are potentially involved. A major investment includes "high-type highway improvements of substantial cost that are expected to have a significant impact on capacity, traffic, level of service, or mode share at the transportation or sub-area scale" (2).
  • An environmental impact statement (EIS) or environmental assessment will have to be undertaken if the proposed improvement is expected to have significant environmental impacts.
  • If the urban area is in non-conformance with air quality standards, a conformity analysis must be undertaken to show no additional degradation of air quality due to the proposed improvement.

The proposed roadway improvements should also be correlated with State and Regional Long-Range Plans, TIPs, etc. As these improvements are generally considered capital projects, they may already be programmed or budgeted; or the proposed improvement might be readily incorporated into another programmed capital project in the same geographic area. Other procedural considerations include:

  • Stakeholders should include those agencies, departments, and staff responsible for designing, building, and maintaining roadway improvements.
  • Performance measures (discussed in Chapter 4) should include indices for identifying locations with recurrent congestion and safety problems, and for determining the nature of the problems and severity.
  • When analyzing and making decisions regarding potential improvements, it is important to remember that changes in traffic and operational patterns resulting from roadway improvements often have an impact that goes beyond the immediate facility that is being improved. For example, additional capacity provided in one corridor could very well influence demand on adjacent arterials or nearby freeways, while eliminating a bottleneck at one location could exacerbate congestion somewhere downstream. Such issues must be considered in the analysis that precedes a decision to improve the roadway and increase capacity. In other words, when looking at alternative roadway improvements, the concept of systemwide performance (as compared to segmental throughput) must be the foremost consideration. As discussed in chapter 4, simulation models have proven very useful in this regard.
  • Geometric design standards contribute to improved freeway safety and throughput. These criteria and guidelines should be incorporated into the design of roadway improvements. Applicable references include (but are not limited to) the AASHTO "Policy on Geometric Design of Highways and Streets" (Reference 5), the AASHTO "Highway Safety Design and Operations Guide" (Reference 6), and the TRB "Geometric Design and the Effects of Traffic Operations" (Reference 7). It is also noted that, at the time of writing this Handbook, ITE is preparing a Freeway and Interchange Geometric Design Handbook.

5.2.2 Relationship to Other Freeway Management Activities

Roadway improvements and more traditional operations improvements – including ITS-based solutions - should work in concert with one another. Moreover, roadway improvements often enhance the application of the strategies addressed in subsequent chapters. For example:

  • Ramp metering (as discussed in Chapter 7) may be more effective, particularly with respect to minimizing back-ups onto the surface street network and / or providing preferred treatment for HOVs, by widening the ramp to provide additional lane(s) or HOV bypass lanes.
  • Truck restrictions (as discussed in Chapter 8 on Managed Lanes) on upgrades may require the addition of a climbing lane to enhance the operation.
  • Incident management (Chapter 10) may be enhanced by the construction of wider shoulders or refuge areas for disabled vehicle.
  • During special event management (Chapter 11) and major evacuations (Chapter 12), the use of shoulders as travel lanes – thereby increasing roadway capacity – may be an appropriate and effective strategy.

5.3 Horizontal and Vertical Alignment

Horizontal and vertical alignments are considered "permanent design elements" (5). It is extremely difficult and costly to correct alignment deficiencies after a highway is constructed. Nevertheless, such changes to the roadway infrastructure may prove a cost-effective and possibly necessary solution, particularly if there are safety issues. Projects to improve horizontal and / or vertical alignments are typically not within the realm of "operational improvements", but are considered highway reconstruction (i.e. the preservation leg of the aforementioned 3-legged stool).

The ITE "Toolbox for Alleviating Traffic Congestion and Enhancing Mobility" (2) defines highway reconstruction as the process of replacing or rehabilitating a road. Reconstruction projects include modernizing geometric and structural standards, improving the quality of operation and safety, increasing capacity, and extending the life of facilities. Importantly, the reconstruction of a facility provides an opportunity to correct or improve operational problems that developed since the facility was built. These improvements could include changes in alignment, improved interchange design, new interchanges, and widening.

While not typically in the realm of "operations", these actions impact freeway operations and should be considered as part of the "toolkit" the practitioner draws from when analyzing and recommending actions to address operational deficiencies. Freeway management practitioners should also be cognizant of and, to the greatest extent possible, directly involved in the planning and design of major reconstruction projects to:

  • Ensure that the projects address identified deficiencies in freeway operation
  • Ensure that disruption to the traveling public is minimized through carefully planned and coordinated construction sequences and work zone management strategies, including the implementation of transportation management strategies and technologies minimize this disruption during reconstruction (e.g., incident management, surveillance, traveler information)
  • Identify the potential impact of reconstruction on existing field components of an ITS-based freeway management system (e.g., conduit and communications cable, CCTV, detectors), and prepare plans for keeping these components operational during reconstruction)
  • Identify elements (e.g., conduit network, foundations) that can be provided in the reconstruction design for system implementation at a later date.

5.4 Roadway Widening

The number of lanes on a freeway segment influences congestion and safety. Widening a freeway to provide additional lanes over several miles falls into the category of a major reconstruction. There are also "bottleneck" situations (i.e., insufficient capacity for just a short distance) where a low-cost roadway improvement can add lanes to eliminate these constraints.

5.4.1 Auxiliary Lanes

An auxiliary lane is defined by AASHTO (5) as the portion of the roadway adjoining the traveled way for speed change, turning, weaving, truck climbing, maneuvering of entering and leaving traffic, and other purposes supplementary to through-traffic movement. Auxiliary lanes are used to balance the traffic load and maintain a more uniform level of service on the highway. They facilitate the positioning of drivers at exits and the merging of drivers at entrances. AASHTO (5) provides the following guidance regarding auxiliary lanes:

  • The width of an auxiliary lane should be equal to the through lanes. Where auxiliary lanes are provided along freeway main lanes, the adjacent shoulder should desirably be 2.4 to 3.6m [8 to 12 ft] in width, with a 1.8m [6 ft] wide shoulder being the minimum considered.
  • Operational efficiency may be improved by using a continuous auxiliary lane between the entrance and exit terminals where interchanges are closely spaced; the distance between the end of the taper on the entrance terminal and the beginning of the taper on the exit terminal is short; and/or local frontage roads do not exist. An auxiliary lane may be introduced as a single exclusive lane or in conjunction with a two-lane entrance.
  • When interchanges are widely spaced, it might not be practical or necessary to extend the auxiliary lane from one interchange to the next. In such cases, the auxiliary lane originating at a two-lane entrance should be carried along the freeway for an effective distance beyond the merging point. An auxiliary lane introduced for a two-lane exit should be carried along the freeway for an effective distance in advance for the exit.

Figure 5-1 illustrates an example of adding an auxiliary lane (8). The Dallas district of the Texas Department of Transportation, in conjunction with the City of Richardson, TX, developed and implemented this solution to improve merging / weaving at the entrance to southbound US 75 from the recently constructed President George Bush Turnpike. The cause of the bottleneck was a forced merge of the ramp traffic onto the southbound main lanes of I-75. Texas Transportation Institute (TTI) conducted an evaluation. Before and after data established that each vehicle using the ramp connection averaged one minute in travel time savings, with a peak savings of over three minutes. At the same time, main lane traffic maintained or experienced a slight increase in speed. The TTI report (6) further states that, in general, the benefit-to-cost ratio for these types of projects are typically high, averaging 20:1 for a ten-year life.

two diagrams of a four-lane freeway section, one before and one after the addition of an auxiliary lane between two closely-spaced entrance ramps that precede an exit ramp

Figure 5-1: Example of Adding an Auxiliary Lane (Reference 8)

5.4.2 Speed-Change Lanes

Drivers leaving a freeway at an interchange are required to reduce speed as they exit on a ramp. Drivers entering a freeway accelerate until the desired highway speed is reached. Because the change in speed is usually substantial, AASHTO (5) recommends that provision should be made for acceleration and deceleration to be accomplished on auxiliary lanes to minimize interference with through traffic and to reduce crash potential. Such an auxiliary lane, including tapered areas, may be referred to as a speed-change lane. The terms "speed-change lane," "deceleration lane," or "acceleration lane", as used in Reference 5, apply broadly to the "added lane joining the traveled way of the freeway with that of the turning roadway and do not necessarily imply a definite lane of uniform width. This additional lane is a part of the elongated ramp terminal area."

A speed-change lane should have sufficient length to enable a driver to make the appropriate change in speed between the freeway and the turning roadway in a safe and comfortable manner. Moreover, in the case of an acceleration lane, there should be additional length to permit adjustments in speeds of both through and entering vehicles so that the driver of the entering vehicle can position himself opposite a gap in the through-traffic stream and maneuver into it before reaching the end of the acceleration lane. This latter consideration also influences both the configuration and length of an acceleration lane (5).

5.4.3 Climbing Lanes

Per AASHTO (5), climbing lanes offer a comparatively inexpensive means of overcoming reductions in capacity and providing improved operation where congestion on grades is caused by slow trucks in combination with high traffic volumes. Although typically applied in rural areas, there are many instances where climbing lanes are needed and appropriate for urban areas. Criteria presented in AASHTO (5) include the following:

  • Critical length of grade – this is the length of a particular upgrade that reduces the speed of low-performance trucks 15 km/h (10 mph) below the average running speed of the remaining traffic. Charts for determining this critical length of grade are provided in the reference (e.g., just under 1000 ft for a 5% upgrade grade). If the critical length of grade is less than the length of grade being evaluated, consideration of a climbing lane is warranted.
  • Existence of a low level of service on the grade. Generally, climbing lanes should not be considered unless the directional traffic volume for the upgrade is equal to or greater than the service volume for LOS D.
  • The location where the climbing lane should begin depends on the speeds at which trucks approach the grade and the extent of sight distance restrictions on the approach. Where there are no sight distance restrictions or other conditions that limit speeds on the approach, the added lane may be introduced on the upgrade beyond its beginning because the speed of trucks will not be reduced beyond the tolerable level to following drivers until they have traveled some distance up the grade. The ideal design is to extend the climbing lane to point above the crest, where a typical truck could attain a speed that is within 15 km/h (10 mph) of the speed of other vehicles. Climbing lanes on multilane roads are usually placed on the outer or right-hand side of the roadway.

5.4.4 Widening Without Adding Lanes

Widening the roadway, but not adding lanes can also improve operations. The potential for an increase in capacity and improved safety (i.e., providing a safe refuge for disabled vehicles) via shoulder widening has already been mentioned.

AASHTO (5) discusses widening the traveled way on horizontal curves to make operating conditions on curves comparable to those on tangents. On earlier highways with narrow lanes and sharp curves, there was considerable need for this widening. On modern highways (12 ft lanes) and high-type alignment, the need for widening has lessened considerably. But for some conditions of speed, curvature, and width, it remains appropriate to widen travel ways. Widening is needed on certain curves for one of the following reasons:

  • The design vehicle occupies a greater width because the rear wheels generally track inside front wheels (off tracking) in negotiating curves, or
  • Drivers experience difficulty in steering their vehicles in the center of the lane.

Design values are provided in this reference for various values of roadway width, design speed, radius of curve, and design vehicles. A minimum widening of .6 m / 2 ft is recommended (5).

5.5 Providing Additional Lanes Without Widening the Freeway

Using freeway shoulders as travel lanes has occurred in some cities since the late 1960s, with many of these lanes being devoted to HOV use. These modifications include using one or more shoulders as travel lanes (this is often done only during peak hours and in the peak direction); and reducing lanes widths to provide additional lanes within the existing pavement. The following discussion of this strategy is taken from the ITE "Toolbox for Alleviating Congestion and Enhancing Mobility" (2).

5.5.1 Benefits/Cost

Significant increases in capacity (up to as much as 30 percent and more) are possible. These capacity increases however, have often been achieved with some increase in accident rates. Thus, the design of such lanes must clearly take into consideration the safety aspects of the particular freeway section. Even though such treatments should be considered temporary, an FHWA staff study found that in cities with populations over one million, almost 32 percent of the urban freeway mileage could experience reduced congestion though such low-cost measures.

A 1995 study of freeway shoulder lanes (Reference 9) found:

  • Freeway capacity in excess of 2,200 passenger car per hour per lane were observed at these sites.
  • Modified sites have a greater tendency to fall into more congested conditions at high volumes than unmodified sites.
  • The range of observed speeds along an unmodified freeway section will be somewhat greater than along a comparable modified section.
  • Accident rates at modified freeway actions are somewhat higher than rates for unmodified sections.
  • Truck accident rates are almost always higher on modified sections.

Another study (Note: Chen, C. “Evaluation of HOV and Shoulder Lane Travel Strategy for I-95”, ITE Journal, September 1995) examined the northern Virginia I-95 use of shoulder lanes for the entire day. This 8 mile / 12.9 km section of Interstate has a left lane designated for 3+ HOV vehicles, two general purpose lanes, and a right shoulder which is used as a conventional travel lane. This study concluded:

  • The use of shoulder lanes increased freeway capacity significantly. Analysis indicated that removing the shoulder lanes from general purpose use would increase queue lengths by 140 percent and system delays by 929 percent. The HOV and shoulder lanes carried 47 percent of road vehicles and 63 percent of total travelers on the freeway.
  • No adverse impacts on general traffic accident frequency were found. Fatality rates were lower than the "before" situation.
  • Importantly, in keeping with the concern mentioned earlier about safety, several modifications were made in the corridor to maintain operational and enforcement activities. In particular, emergency pullouts were built and signed to allow for safe storage of disabled vehicles.

The primary advantages and disadvantages in implementing this tool are summarized below (from Reference 9).

Design Alternative Advantages Disadvantages

Use of Left Shoulder

Left shoulder not used as much for emergency stop or emergency enforcement.

Lease expensive if width is available.

Trucks often restricted from left lane.

Usually requires restriping.

Sight distance problem with some median treatments.

Use of Right Shoulder

Often the easiest to implement.

Right shoulder is preferred area for emergency stops and enforcement.

Sight distance changes are merge and diverge areas of ramps.

Use of Both Shoulders

Not recommended.

Use only in extreme cases.

Requires restriping

Safety concerns

Enforcement difficult

Incident response longer

Maintenance more difficult and expensive

5.5.2 Implementation

Whenever improvements are made to a highway, the level of safety should be improved. As noted in the AASHTO 1997 "Highway Safety Design and Operations Guide":

"The need to accommodate more traffic within existing or limited additional right-of-way on high volume urban freeways has led some agencies to increase capacity by exchanging full-lane or shoulder widths for additional travel lanes with reduced widths. Any proposed use of less than full standard cross section must be studied carefully on a case-by-case basis. Experience indicates that 12ft- lanes can operate safely if there are no other less-than-standard features; however, combined with shoulder width reductions, substandard sight distance, and other features, (these) lanes may not provide the same operation"

This means that when shoulder use is being considered for traffic flow, careful planning and design should occur to avoid any potential safety problems. In addition, structural capacity of a highway varies across the cross section. The shoulder is not often constructed to accommodate traffic loads. Pavement failures and subsequent repair under traffic conditions will have an effect on both capacity and safety.

Cooperation and coordination between the state highway agency and the traffic enforcement officials responsible for enforcement (i.e., "stakeholders") is essential. Because the use of breakdown lanes is not consistent with federal design criteria, federal approval will be required if the highway facility is on the federal-aid system.

When this action is being considered, it typically generates opposition from traffic enforcement agencies and motorists who are mainly concerned about safety (i.e. the emergency lane is used for traffic flow rather than by emergency vehicles or breakdowns). Also, there is concern that the flow from entrance ramps will be adversely affected. There are all legitimate concerns that should be addressed. The response to these concerns includes the following from the AASHTO 1997 "Highway Safety Design and Operations Guide" (Reference 6):

  • Where shoulders are converted to travel lanes, removing the left-side shoulder is preferable.
  • Horizontal and vertical curvature should be checked for adequate stopping sight distance, particularly in those instances where the median shoulder is being considered as a travel lane, and the highway has a narrow median and median barrier.
  • Systems for rapid incident detection and response should be considered for sections with substandard lanes and shoulder widths. (This includes removing disabled vehicles from the shoulders before the peak period when the shoulder becomes a traveled lane).
  • If both shoulders are removed; mitigating measure should include adequate advisory and regulatory signing, constructing frequent emergency pullouts, active overhead and side-mounted changeable message signs and signals, continuous lighting, truck lane use restrictions, dedicated service patrols and continuous enforcement.
  • For sections greater than 1.5 kms where inadequate shoulders are provided emergency pullouts should be considered where feasible.

5.6 Interchanges

Interchanges are where traffic enter and exit the freeway. The merging and weaving associated with interchanges affect traffic flow. Improvements can be made to increase the capacity and safety of the weaving sections and the ramps that comprise the interchange.

5.6.1 Weaving Segments

The Highway Capacity Manual (1) identifies three geometric variables that influence weaving segment operations (i.e., configuration, length, and width) as discussed below:

  • Configuration – The configuration of the weaving segment (i.e., the relative placement of entry and exit lanes) has a major effect on the number of lane changes required of weaving vehicles to successfully complete their maneuver.
  • Weaving Length – Because weaving vehicles must execute all the required lane changes for their maneuver within the weaving segment boundary from the entry gore to the exit gore, the parameter of weaving length is important. The length of the weaving segment constrains the time and space in which the driver must make all required lane changes. Thus, as the length of a weaving segment decreases (configuration and weaving flow being constant), the intensity of lane changing, and the resulting turbulence, increase. Similarly, by increasing the length of the weaving area, capacity is increased.
  • Weaving Width – The third geometric variable influencing the operation of the weaving segment is its width, which is defined as the total number of lanes between the entry and exit gore areas, including the auxiliary lane, if present. As the number of lanes increases, the throughput capacity increases. At the same time, the opportunity for lane changing also increases for discretionary lane changes that may take place within the weaving segment.

Another variable is volume. The weaving geometrics of an interchange may work quite well under one combination of through and entering / exiting volumes, and lead to congestion and safety problems under another (e.g., higher volumes).

Depending on the interchange layout and the distances between adjacent interchanges, capacity may be increased, safety improved, and weaving operations improved by the addition of auxiliary lanes and other widening efforts as previously discussed.

5.6.2 Ramp Components

The term "ramp" is used by AASHTO (5) to include all types, arrangements, and sizes of turning roadways that connect two or more legs on an interchange. Figure 5-2 illustrates several types of ramps and their characteristic shapes. Various configurations are used; however each can be broadly classified as on of the types shown. The different ramp patterns of an interchange (i.e., the different types of interchange configurations) are made up of various combinations of these types of ramps.

five diagrams of different types of freeway ramps: diagonal, one quadrant, loop and semi-direct, outer connection, and directional

Figure 5-2: Types of Freeway Ramps (Reference 5)

There are a number of variables that influence the operation of ramp-freeway junctions. They include all of the attributes affecting basic freeway segment operation (e.g., number of lanes and lane widths, lateral clearances, terrain and grades, degree of curvature) There are additional parameters of particular importance to the operation of ramp-freeway junctions, including length and type (taper, parallel) of acceleration/deceleration lanes, sight distances, speed, and lane distribution and free flow speeds of upstream freeway traffic.

The length of the acceleration or deceleration lane has a significant effect on merging and diverging operations. Short lanes provide on-ramp vehicles with restricted opportunity to accelerate before merging and off-ramp vehicles with little opportunity to decelerate off-line. The result is that most acceleration and deceleration must take place on the mainline, which disrupts through vehicles. Short acceleration lanes also force many vehicles to slow significantly and even stop while seeking an appropriate gap in the traffic stream. The free flow speed of the freeway is also an influential factor, since it determines the speed at which merging vehicles enter the acceleration lane and the speed at which diverging vehicles must enter the ramp. This, in turn, determines the amount of acceleration or deceleration that must take place.

Like the freeway, enhancements to many of these ramp parameters (e.g., length, grade, curvature, sight distance) will require major reconstruction. Other ramp and interchange attributes – such as number of ramp lanes and lane widths, and the length of acceleration and deceleration lanes – may be modified through relatively low cost projects (similar to those discussed above for the freeway) to improve operations.

5.7 Other Improvements

Certain types of collisions may be reduced and / or their severity lessened by the implementation of specific corrective measures. This section provides an overview of some of theses that may be considered in the overall context of freeway management and operations.

5.7.1 Roadside Obstacles

AASHTO (5) recommends the following priorities for treatment of roadside obstacles on existing highways:

  • Remove the obstacle or redesign so it can be safely traversed
  • Relocate the obstacle to a point where it is less likely to be struck
  • Reduce the severity of impact with the obstacle using an appropriate breakaway device
  • Redirect the vehicle by shielding the obstacle with a longitudinal traffic barrier and / or crash cushion
  • Delineate the obstacle if the above alternatives are not appropriate

It is noted that the design of guardrail and barrier systems has had considerable research. References include the AASHTO Roadside Design Guide and NCHRP Report 350 ("Recommended Procedures for the Safety Performance of Highway Features"). These publications note that the treatment of end sections of guardrail or a barrier is of particular concern.

5.7.2 Skid Resistance

Skidding crashes are a major concern in highway safety. It is not sufficient to attribute skidding crashes merely to "driver error" or "driving too fast for existing conditions." The roadway should provide a level of skid resistance that will accommodate the braking and steering maneuvers that can reasonably be expected for the particular site.

Highway geometrics affect skidding. Therefore, skid resistance should be a consideration in the design of all new constructions and major reconstruction projects. Vertical and horizontal alignments can be designed in such a way that the potential for skidding is reduced. Also, improvements to be vertical and horizontal alignments should be considered as a part of any reconstruction project (5).

Pavement types and textures also affect a roadway's skid resistance. The four main causes of poor skid resistance on wet pavement are rutting, polishing, bleeding, and dirty pavements. Rutting causes water accumulation in the wheel tracks. Polishing reduces the pavement surface microtexture and bleeding can cover it. In both cases, the harsh surface features needed for penetrating the thin water film are diminished. Pavement surfaces will lose their skid resistance when contaminated by oil drippings, layers of dust, or organic matter. Measures taken to correct or improve skid resistance should result in the following characteristics: high initial skid resistance durability, the ability to retain skid resistance with time and traffic, and minimum decrease in skid resistance with increasing speed (5).

Tining during placement leaves indentations in the pavement surface and has provided to be effective in reducing the potential for skidding on wet roadways with Portland cement concrete surfaces. The use of surface courses or overlays constructed with polish-resistant coarse aggregate is the most widespread method for improving the surface texture of bituminous pavements. Overlays of open-graded asphalt friction courses are quite effective because of their frictional and hydraulic properties. For further discussion, refer to the AASHTO Guidelines for Skid Resistant Pavement Design. Grooving

Another potential solution to correcting a pavement surface with low skid resistance may be modification of the existing surface rather than the application of a new surface. Grooving is a technique of altering the existing pavement surface to greatly increase the texture, thereby facilitating the displacement of water by tires. When a tire rolls over a rain-soaked surface, water forms a layer between the tire and pavement, and a portion of the tire is raised off the surface. The water pressure increases with the speed of the vehicle, lifting more and more of the tire, until the tire loses contact with the pavement. This situation – where the tire (and vehicle) floats above the road surface on a layer of water – is known as hydroplaning. If the tire is not actually touching the road or runway surface, it's impossible to steer or brake, significantly increasing the chance for a crash or other accident. The grooved pavement provides escape routes for water compressed between the tire tread and the road. The grooves restore friction performance to worn or smooth pavement surfaces, enhancing braking and cornering under wet conditions because more of the tire's surface is on the pavement rather than on water. Various specifications for grooving can be found on the website for the International Grooving and Grinding Association (

In addition to the often-dramatic reduction in wet weather accidents, grooving can be accomplished with minimal disruption to traffic – only one lane needs to be closed at a time, and traffic can use the grooved pavement shortly thereafter. There are some potential disadvantages to grooving, including (5):

  • Grooving can't be used for bituminous concrete unless the asphalt is well cured; otherwise spalling of the grooves occurs
  • Use of studded ties and chains reduces the service life of the grooved pavement
  • Motorcyclists and drivers of small cars may have a sensation of instability

It is also noted that while grooving reduces wet weather accidents, the pavement skid resistance as measured by conventional means does not increase significantly.

5.7.3 Emergency Escape Ramps

The AASHTO publication "A Policy on Geometric Design of Highways and Streets" (5) discusses emergency ramps at length. The following is a summary taken from the AASHTO document.

  • Need for Emergency Escape Ramps – Each grade has its own unique characteristics. Highway alignment, gradient, length, and descent speed contribute to the potential for out-of-control vehicles. For existing highways, law enforcement officials, truck drivers, or the general public will often report operational problems on a downgrade. A field review of a specific grade may reveal damaged guardrail, gouged pavement surfaces, or spilled oil indicating locations where drivers of heavy vehicles had difficulty negotiating a downgrade.
  • Location of Escape Ramps – Ramps should be located to intercept the greatest number of runaway vehicles, such as at the bottom of the grade and at intermediate points along the grade where an out-of-control vehicle could cause a catastrophic crash. Escape ramps generally may be built at any practical location where the main road alignment is tangent. They should be built in advance of horizontal curves that cannot be negotiated safely by an out-of-control vehicle and in advance of populated areas. Escape ramps should exit to the right of the roadway.
  • Design Considerations – The alignment of the escape ramp should be tangent or on very flat curvature to minimize the driver's difficulty in controlling the vehicle. The width of the ramp should be adequate to accommodate more than one vehicle because it is not uncommon for two or more vehicles to have need of the escape ramp within a short time. The entrance to the ramp should be designed so that a vehicle traveling at a high rate of speed can enter safely - an auxiliary lane may be appropriate to assist the driver to prepare to enter the escape ramp. Access to the ramp should be made obvious by advance signing to allow the driver of an out-of-control vehicle time to react, so as to minimize the possibility of missing the ramp. Regulatory signs near the entrance should also be used to discourage other motorists from entering, stopping, or parking at or on the ramp.

5.8 References

1. Highway Capacity Manual, Transportation Research Board, National Research Council, Washington D.C: 2000

2. Meyer, M.D. A Toolbox for Alleviating Traffic Congestion and Enhancing Mobility. Institute of Transportation Engineers, Washington D.C. 1997

3. Homburger, W.S., Hall, J.W., Reilly, W.R. and Sullivan, E.C. Fundamentals of Traffic Engineering – 15th Edition. University of California, Berkley (UCB-ITS-CN-01-1), January 2001.

4. Effectiveness of Alternative Skid Reduction Measures, FHWA, RD-79-25, November 1978

5. A Policy on Geometric Design of Highways and Streets, American Association of State Highway and Transportation Officials, Washington, D.C. 2001

6. Highway Safety Design and Operations Guide, American Association of State Highway and Transportation Officials, Washington, D.C. 1997

7. Geometric Design and the Effects on Traffic Operations, Transportation Research Record 1796, Washington D.C., 2002

8. "Do Bottleneck Improvements Really Reduce Congestion?", Texas Transportation Researcher, TTI, 2002

9. NCHRP Report 369 "Use of Shoulders and Narrow Lanes to Increase Freeway Capacity"