If you mean "safety impact", then look for crash modification factors - http://www.cmfclearinghouse.org/

Hello,

Thank you for the answer. However, I'm just looking for a cost estimate method to calculate the cost of turning a park lane into shoulder. It's a residential area, so people live in that area park their cars there. I was wondering how to turn this impact, into a cost estimation.

There are many local government schemes which hire on-street car parking spaces to private uses, e.g. during building construction, or long-term e.g. Flexicar, or GoGet. YOu could look those up and use as the high-end baseline. Willingness-to-pay is alos an option, i.e. asking the people in the area what is of comparative value (amenity/service). Good luck. It's a tough one. 

The purpose of a shoulder is different from a parking lane. You mentioned that the neighborhood is residential. The existence of the parking lane indicated that it been an on-street parking in a high density area. First you need to consider the posted speed of the local street and the need of the proposed shoulder. Was the striping of the existing parking diagonal or longitudinal? If it is diagonal, it can fit more vehicles than longitudinal. As you may be aware, the shoulder could be meant for emergency stopping alone and not for parking overtime. Relative to the cost impact and based on your local currency, consider the displacement of the estimated number of vehicles. This will require cost of acquiring an open land or purchase of existing property for the purpose of development to provide parking within the neighborhood for the displaced vehicles (including cost of development), or an estimated parking charge at existing private parking lot to the displaced owners within the neighborhood. You should conduct a field survey to determine the cost of daily parking in a nearby downtown or central city near the area and estimate the annual cost of parking each vehicle to the respective owner. Multiply by the total vehicles displaced. You could pre-determine an interest rate for the cost impact over 10 or more years. You need to determine the justification of converting the parking lane to shoulder relative to its impact to the municipality/public, current traffic condition, as well as the owners of displaced vehicles.

It all depends on, what you mean by a cost estimate. In general, you should do a normal cost-benefit analysis and include in it all the costs (construction and maintenance, user costs, including accident costs, time costs, environmental costs,etc.)  in both cases. Usually parking lane has safety effects, which are not at all straight forward to estimate. Parking can lower the driving speeds and thus give additional safety. On the other hand, it causes safety problems when cars are parked or leave from parking. Moreover, there are environmental problems of parked cars (noise, pollution, visual deteroriation), people do not necessary like cars parked on the street. In many cities, the basic parking rule is, that parking on the streets is not allowed, so in addition to cost-benefit considerations, this is a policy question, too.