All ramps and connections should be designed to enable vehicles to leave and enter the traveled way of the freeway at 85 percent (desirable) to 70 percent (usual) of the freeway's design speed, rounded up to the nearest 5 mph increment, and limiting the speed differential to 10 mph on the upper range and 20 mph for the mid-range. The upper and mid-range design speeds are not always practical (e.g., constrained ROW or retrofit to existing frontage roads, etc.) and lower design speeds may be selected upon review from Design Division, but they should not be less than the lower range presented in However, every effort should be made to meet the desirable ramp/connector design speed. These speeds do not pertain to ramp terminals, which should be properly transitioned and provided with speed-change facilities adequate for the highway speed involved.

Where the ramp joins a frontage road, the design speed over the length of the ramp may vary, with the portion of the ramp closer to the frontage road being designed to the frontage road speed (i.e., the lower speed).

The independent design speed of the ramp should normally be assumed to be from the end of one physical nose to the beginning of the opposite physical nose.

See for the definition of gore area characteristics.

The design speed for a ramp should not be less than the design speed on the intersecting frontage roads. Refer to for additional guidance on the application of the ranges of ramp design speed shown in . Refer also to Guides for more detailed analysis.

The horizontal geometry of entrance ramps will depend on the design speeds of the ramp, the connecting frontage road, and the connecting freeway. The departure from the frontage road should be a smooth transition appropriate for the design speed and speed classification (low speed/high speed) of the frontage road. The pavement elevation and cross slope of the departing ramp should match the pavement elevation and cross slope of the connecting frontage road up to the physical nose. Once the ramp is independent of the connecting frontage road, the ramp horizontal geometry and cross slope should be set to the design speed of the ramp. Where the ramp connects to the freeway, beyond the physical nose, the pavement elevation and cross slope of the entering ramp should match the pavement elevation and cross slope of the connecting freeway.

The horizontal geometry of the exit ramps will depend on the design speeds of the ramp, the connecting freeway, and the connecting frontage road. The departure from the freeway may be either tangent or parallel. The pavement elevation and cross slope of the departing ramp should match the pavement elevation and cross slope of the connecting freeway up to the physical nose. Once the ramp is independent of the connecting freeway, the ramp horizontal geometry and cross slope should be set to the design speed of the ramp. Where the ramp connects to the frontage road, beyond the physical nose, the pavement elevation and cross slope of the exiting ramp should match the pavement elevation and cross slope of the connecting frontage road.

The term “gore” indicates an area at the junction of entrance or exit ramps with the mainlane or frontage roads which is illustrated in

For an exit ramp junction at the mainlane, the physical nose is a point downstream from the painted nose of the gore, having some dimensional width that separates the roadways. The painted nose is a point, with no dimensional width, occurring where the travel ways intersect for the mainlane and ramp. The neutral area refers to the triangular area between the painted nose and physical nose.

The geometric layout of these is an important part of exit ramp terminal design. It is the decision point area that should be clearly seen and understood by approaching drivers and it should have a geometric shape appropriate for the likely speeds at that point. In a series of interchanges along a freeway, the gores should be uniform and have the same appearance to drivers.

The gore area, and the unpaved area beyond, should be kept as free of obstructions as practical to provide a clear recovery area. The unpaved area beyond the physical nose should be graded to be as nearly level with the roadways as practical so that vehicles inadvertently entering will not be overturned or abruptly stopped by steep slopes.

For design controls for crest and sag vertical curves on ramps and direct connectors, see and . Longer vertical curves with increased stopping sight distances should be provided wherever possible. See for guidance on grade breaks without a vertical curve.

The tangent or controlling grade on ramps and direct connectors should be as flat as possible, and

preferably should be limited to 4 percent or less.

Maximum ramp grades are shown in . In general, these grades should be used infrequently and only when necessary. Engineering judgement should be exercised when there are certain geometric constraints or topographic conditions that permit the use of these maximum grades.

The ramp should have an independent profile from physical nose to physical nose. The profile grade within the neutral area should be designed as a projection of the connecting road profile and cross slope. shows typical characteristics of ramp profile limits.

In some cases, it may be impractical to match the projected roadway profile and cross slope when designing the ramp proper profile. For these situations, a separated median area with mountable curb or barrier that warps the neutral area to join the projected profile of the connecting roadway and ramp proper may be considered. If a raised median is not preferred, surfaced mounted delineators may be used within the warped neutral area to discourage drivers from traversing this part of the ramp. See for illustrations of a warped neutral area and for maximum algebraic differences in cross slope.

Superelevation rates, as related to curvature and design speed of the ramp or direct connector, should be based on values seen in or

The cross section of a ramp or direct connector is a function of the following variables:

Lane and shoulder widths for ramps and direct connectors are shown in

Sufficient sight distance should be provided for the probable vehicle speeds on ramps and direct connectors taking into consideration the grade, vertical curve, alignment, and lateral and corner obstructions to vision. Sight distance and sight lines are especially important at merge points for ramps and mainlanes or between individual ramps. See for recommended stopping sight distances for ramps and direct connections.

The sight distance on a freeway preceding the approach nose of an exit ramp should exceed the minimum stopping sight distance for the freeway design speed, preferably by 25 percent or more. For

Decision Sight Distance

, see , for a desirable goal.

The design of ramp terminals for exits from and entrances onto freeways and frontage roads has a significant impact to safety, operation, and capacity of the freeway facility. The following ramp terminal design features are discussed:

The acceleration lane associated with entrance ramps to freeway begins where the driver transitions from the ramp curvature to the flatter geometry of the speed change lane (SCL). The lane allows for acceleration and checking for gaps in the freeway traffic. The length of the acceleration lane is determined on the basis that merging vehicles should enter the through lane at a speed approximately equal to the running speed of the freeway. The acceleration lane ends at the beginning of the taper. The taper section begins where the width of the SCL becomes less than 12-ft and ends at the point where the SCL has fully merged with the freeway through lane. The taper section is not included in the length of the SCL. The factors for the calculation of the length of an acceleration lane are:

See for minimum acceleration lane lengths (La) for entrance terminals applicable to both taper- and parallel-type entrances. Adjustment factors for grade effects are shown in . Point A (shown in ) is the point where La is measured from and controls speed on the ramp. La should not start back on the curvature of the ramp unless the radius of the ramp equals 1,000-ft or more.

Before La is extended back onto the ramp past the physical nose of the gore, the actual vertical and horizontal alignments of the ramp should be analyzed to determine if acceleration above the ramp design speed can reasonably begin there.

Gap acceptance length (Lg) is defined as beginning where the traveled ways of the mainlane and ramp roadways are a minimum of 2 lateral feet apart and ending where the right edge of the ramp is 12-ft from the right edge of the through lane of the freeway. The minimum gap acceptance length is 300-ft. The value of La or Lg, whichever produces the greater distance, is suggested for use in the design of the ramp entrance.

The two general forms of entering a freeway are through a taper type and parallel type speed change lanes.

A taper-type entrance shown in merges into the freeway with a long, uniform taper. A desirable rate of taper is approximately 50:1 to 70:1 (Design Speed:1 or DS:1) between the outer edge of the acceleration lane and the edge of the through traffic lane. The rate of taper is measured against the alignment of the mainlanes.

The geometrics should be designed in a way that drivers attain a speed that is within 5 mph of the operating speed of the freeway by the time they reach the point where the left edge of the acceleration lane joins the traveled way of the freeway.

The parallel-type entrance ramp to freeway shown in provides an added lane of sufficient length to enable a vehicle to accelerate to near-freeway speed prior to merging. A taper is provided at the end of the added lane.

Desirably, a ramp curve with a baseline radius of 1,000-ft or more and length of at least 200- ft should be provided in advance of the added lane. If a radius less than 1,000-ft is used drivers tend to drive directly onto the freeway without using the acceleration lane. The taper at the downstream end of a parallel-type acceleration lane should be a suitable length to guide the vehicle gradually onto the through lane of the freeway. A minimum taper length of 300-ft is required.

The length of a parallel-type acceleration lane is generally measured from the point where the left edge of the ramp travel way joins the traveled way of the freeway to the beginning of the downstream taper, Lg. Acceleration usually takes place downstream from this point. However, a part of the ramp may also be considered in the acceleration length, La, provided the ramp curve approaching the acceleration lane has a radius of 1,000-ft or more and the motorist on the ramp has an unobstructed view of traffic on the freeway to the motorist’s left.

An acceleration length, La, of at least 1,200-ft plus the taper is desirable where the ramp-freeway volume is nearing 4,600 vehicles per hour at the ramp-freeway junction.

Connecting taper-type entrance ramps to freeways in a horizontal curve will need to be independently evaluated depending on the radius of the mainlanes as well as any superelevation and/or direction of the cross slope of the mainlanes, if any. Refer to for guidance on developing the alignment of a speed change lane on curves.

through show different cross slope and curvature combinations of an entrance ramp connecting to the mainlanes. In all scenarios, the rate of taper (DS:1) is measured against the alignment of the mainlanes. If the mainlanes are superelevated, the cross-slope transition should occur in the ramp prior to the physical nose. In the case where ramp curves are reverse of mainlanes or ramp cross slope exceed mainlane cross slope, a sufficient length of tangent or a larger compound curve must be provided on the ramp to accommodate cross slope transitions prior to the physical nose. When a taper-type entrance ramp is used in a curve, the taper should be evenly distributed throughout the curve to meet the desirable taper length.

The deceleration lane associated with exit ramps from freeways is sometimes preceded by a tapered section of roadway. It begins where the speed change lane is 12-ft and ends at Point A. The deceleration lane provides a means for vehicles to decelerate from the freeway speed to a speed commensurate with the controlling feature of the ramp. The deceleration lane length (La) is based on three factors: (a) the speed at which drivers maneuver onto the auxiliary lane, (b) the speed at the end of the deceleration lane, and (c) the manner of deceleration.

A taper-type exit ramp, as shown in tangent example begins with an abrupt outer edge alignment break providing a clear indication of the point of departure from the through lanes. The divergence angle is usually 2 to 5 degrees. The angular break may be rounded by an appropriate radius at the design engineer’s discretion.

The taper-type exit may also be designed using an alignment as shown in

curvilinear example.

The divergence angle resulting from the designed alignment should be calculated and compared to the typical 2 to 5 degrees.

A parallel-type exit terminal usually begins with a taper followed by an added lane that is parallel to the traveled way as shown in . In locations where both the mainlane and ramp carry high volumes of traffic, the deceleration lane provided by the parallel-type exit provides storage for vehicles that would otherwise undesirably queue up on the through lane or on a shoulder. The length of a parallel-type deceleration lane is usually measured from the point where the added lane attains a 12-ft width to the point where the alignment of the ramp roadway departs from the alignment of the freeway. Where the ramp between the gores is curved it is desirable to provide a transition at the end of the deceleration lane.

Minimum deceleration lengths for various combinations of design speeds for the highway and for the ramp roadway are given in . For grade adjustments see . The values in for minimum deceleration lane length on exit ramps do not account for any deceleration in the through lanes, therefore providing a conservative estimate for design. The designer should assume that all deceleration takes place in the speed-change lane when determining the minimum deceleration lane length.

The geometry of ramps connecting from frontage roads will differ significantly depending on whether they are connecting to a one-way or two-way frontage road. Connection from one-way frontage roads is recommended, as this reduces the chance of wrong-way traffic on a ramp. The horizontal geometry of the ramp connecting from a frontage road should be equal to or more conservative than the frontage road dependent upon the context and speed, see for appropriate superelevation methodologies.

While treatment of pavement edges may differ depending on whether the frontage road is shouldered or curbed, the path of the traffic remains the same. The traffic path should follow a smooth horizontal curve tangent to both the inside yellow stripe of the frontage road through to the inside yellow stripe of the ramp. This applies whether or not the traffic is entering from the through lanes on the frontage road or a dedicated auxiliary lane. Geometry guidance for entrance ramps from frontage roads is shown in and . For more geometry and access control information on entrance ramps from frontage roads, see .

Ramps should connect to the frontage road at the minimum distance (see ). Greater distances are desirable to provide adequate weaving length, space added for vehicle storage, and turn lanes at the cross street. For more geometry and access control information on exit ramps to frontage roads, see

The minimum acceptable distance between ramps is dependent upon the merge, diverge, and weaving operations that take place between ramps, as well as distances required for signing.

The distance between successive ramps must be determined with the analysis procedures referred to in the

. A design waiver will be required if the minimum distance for the minimum Level of Service (LOS) as determined by an HCM analysis cannot be met. Additionally, a design waiver will be required if the minimum values in cannot be met.

When the minimum needed distance cannot be achieved, consideration should be given to consolidating or braiding the ramps in lieu of requesting a design waiver.

Ramp configurations shown in are relative to the facility being traveled on.

See section 15.8.1 for additional considerations on ramp spacing along frontage roads.

Where ramps are currently expected to accommodate metering, or expected to in the future, the geometric design features referred to in design criteria for ramp metering should be considered. Ramp metering, when properly designed and installed, has been shown to have benefits for the operation of the mainlanes. However, since ramp meters are installed to control the number of vehicles that are allowed to enter the mainlanes, an analysis of the entire roadway network area should be done to determine adverse operational impacts to other roadways. It is suggested that the analysis specifically include both frontage road and adjacent cross street operations of through traffic, turning movements, and queue lengths. Refer to for additional discussion and design details on ramp metering.